I 


SOIL 


i|H;H 


LYON  AND  PIPPIN 


L.  H.  BAI  LEY 

EDITOR 


^  • .  c 


UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


GIFT  OF 

Mrs.   M.   H.   Wood 


Cfje  ftural 

EDITED  BY  L.  H.  BAILEY 


THE  PRINCIPLES  OF   SOIL 
MANAGEMENT 


THE  MACMILLAN  COMPANY 

NEW  YORK    •    BOSTON   •    CHICAGO 
ATLANTA   •    SAN   FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON    •    BOMBAY   •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


Plowing — the  most  fundamental  and  far-reaching  operation 
in  soil  management. 


E  PRINC  PLES 


SOI  L  M AN AGEMEF 


BY 

T.   LYTTLETON   LYON,   Ph.D. 

AND 

ELMER   0.   FIPPIN,   B.S.A. 

PROFESSORS  OF  SOIL  TECHNOLOGY,  IN  THE  NEW  YORK  STATE  COLLEGE 
OF  AGRICULTURE  AT  CORNELL  UNIVERSITY 


FIFTH   EDITION 


lorb 
THE   MACMILLAN   COMPANY 

LONDON:   MACMILLAN  &  CO..  LTD. 

1912 

A.U  rijhtt  reterved 


COPYRIGHT,  1909 
By  THE  MACMILLAN  COMPANY 


Set  up  and  electrotyped. 

Published,  December.  1909 

Reprinted  June,  1910,  January,  November,  1911,  June,  1912 


Mount  pleasant 
J.  Horace  McFarland  Company 
Harrisburtf,  Pennsylvania 


* 
* 
^^ 
V 
\^ 


PREFACE   TO   THE  RURAL 
TEXT  -BOOK  SERIES 

In  1895  the  preface  was  written  for  the  Rural  Science 
Series.  It  set  forth  the  purpose  of  the  Series  to  be  the 
desire  to  place  in  readable  form  the  best  results  of 
scientific  thought  and  discovery  relating  to  agriculture 
and  country  life,  in  order  that  the  general  public  might 
be  made  aware  of  the  progress,  and  that  farmers  might 
be  led  more  effectively  to  apply  the  information  in  their 
daily  work.  It  was  the  hope  that  the  Series,  under  the 
present  writer's  direction  or  another's,  might  gradually 
extend  itself  to  the  whole  range  of  agricultural  scientific 
literature.  The  books  now  included  in  The  Rural  Science 
Series  are  about  two  dozen,  making  nearly  two  volumes, 
on  the  average,  for  each  year.  The  number  of  writers  on 
agricultural  topics  is  increasing,  the  knowledge  on  all 
subjects  is  rapidly  accumulating,  and  the  reading-public 
is  gradually  enlarging;  there  is  every  reason  to  expect, 
therefore,  that  the  Series  will  extend  itself  still  more 
rapidly  in  the  years  to  come. 

It  was  considered  to  be  an  auspicious  circumstance 
that  the  Rural  Science  Series  began  with  a  book  on  the 
soil,  for  this  grounded  the  enterprise.  The  scientific  and 

(v) 

374024 


VI        PREFACE   TO   THE  RURAL   TEXT-BOOK  SERIES 

literary  character  of  this  first  volume  also  won  a  good 
hearing  for  the  undertaking. 

The  time  has  come  when  special  texts  on  agri- 
cultural and  rural  subjects  are  needed  in  educational 
institutions;  and  I  now,  therefore,  project  another  line 
of  rural  books,  to  be  known  as  The  Rural  Text-Book 
Series.  This  Series  is  to  be  coordinate  with  the  other 
Series,  the  former  designed  primarily  for  popular  read- 
ing and  for  general  use,  this  one  for  class-room  work 
and  for  special  use  in  consultation  and  reference.  It  is 
planned  that  the  Rural  Text-Book  Series  shall  cover 
the  entire  range  of  public-school  and  college  texts. 

I  consider  it  to  be  significant  that  I  am  able  to  begin 
this  new  Series,  also,  with  a  book  on  the  soil.  These 
two  soil  books  well  illustrate  the  two  methods  of  treat- 
ment of  a  subject;  and  this  later  one  impels  us  anew  not 
to  forget,  in  all  our  new  discussions,  and  especially  amid 
the  social  and  economic  speculations  on  which  we  are 
now  entering,  that  a  well-maintained  soil  is  the  first 
essential,  not  only  to  agricultural  progress  but  to  human 
prosperity.  The  soil  is  the  greatest  natural  resource. 
We  must  never,  in  our  philosophy,  get  away  from  the 
land. 

Attention  is  called  to  the  analysis  of  the  subject- 
matter  of  this  volume  as  outlined  in  the  table  of  contents 
and  expanded  in  the  text.  The  educational  value  of 
any  subject  or  volume  lies  not  so  much  in  the  information 


PREFACE   TO    THE  RURAL   TEXT-BOOK   SERIES      vii 

that  is  presented  as  in  the  organization  of  the  information 
into  a  systematic  treatment,  whereby  a  philosophy  of 
the  subject  is  developed.  A  college  text  should  be  a 
unity,  rounding  up  the  subject  so  completely  as  to  give 
the  student  a  grasp  of  the  material  as  one  problem, 
and  at  the  same  time  expounding  the  reasons  on  which 
the  treatment  rests.  When  the  student  has  completed 
any  text,  he  should  have  a  clear  mental  topography  of 
the  subject  that  it  treats.  So  may  the  agricultural 
subjects  be  made  the  agencies  in  developing  clear  think- 
ing, sound  argument,  constructive  imagination,  and 
effective  application  to  the  needs  of  life. 

L.  H.  BAILEY. 

Ithaca,  N.  Y. 
October  1,  1909 


AUTHORS'   PREFACE 

In  teaching  introductory  courses  in  soil  technology 
to  agricultural  students,  the  authors  feel  that  the  use  of 
a  text  book  enables  the  student  to  get  a  more  thorough 
mental  discipline  and  a  better  grasp  of  the  details  of  the 
subject  than  can  result  from  a  course  of  lectures.  The 
present  book  is  the  outgrowth  of  their  experience  in 
teaching  soil  technology  through  a  period  of  several 
years.  It  has  been  their  endeavor  to  present  the  appli- 
cation of  science  to  soil  problems  from  the  standpoint 
of  crop-production  rather  than  that  of  any  one  of  the 
underlying  sciences  of  geology,  chemistry,  physics  or 
bacteriology.  This  has  necessitated  drawing  from  a  wide 
range  of  literature,  and  arranging  the  material  in  a  form 
which  it  is  thought  adequately  represents  all  phases  of 
the  subject.  The  sources  of  such  data  have  been  freely 
drawn  upon,  and  the  authors  take  this  opportunity  to 
express  their  obligations  for  the  aid  they  have  received 
from  a  very  large  number  of  papers  and  books  dealing 
with  soils,  and  which  it  has  not  been  found  practicable  to 
credit  specifically  in  the  text,  as  has  been  done  in  many 
instances. 

It  may  happen  that  some  teachers  will  not  wish  to 


X  AUTHORS'  PREFACE 

follow  the  entire  text,  in  which  event  we  think  it  will  be 
found  possible  to  omit  certain  sections  and  yet  have  a 
connected  treatment  of  the  subject.  On  the  other  hand, 
very  little  attempt  has  been  made  to  supply  illustrations 
of  the  principles  which  are  explained.  Such  illustrations 
and  amplifications  are  left  to  be  added  by  the  teacher  as 
local  conditions  and  interests  may  dictate. 

The  book,  as  its  title  implies,  deals  largely  with  the 
Principles  of  Soil  Technology,  and  applications  of  these 
to  local  practice  should  constitute  a  part  of  the  instruc- 
tion. 

Attention  is  called  to  the  outline  of  contents,  which 
shows  the  method  of  treatment  and  the  relation  of  the 
several  parts  of  the  subject.  As  an  elementary  treatise,  it 
has  been  the  aim  to  properly  balance  the  discussion  of 
all  phases  of  the  subject,  which  may  be  followed  in 
greater  detail  in  advanced  courses. 

In  the  illustrations,  endeavor  has  been  made  to 
include  cuts  of  all  of  the  more  common  types  of  soil- 
working  implements.  We  are  indebted  to  the  United 
States  Bureau  of  Soils  for  several  illustrations,  and  to 
Pfeffer's  'Pflangenphysiology'  for  three  cuts  which,  by 
mistake,  were  not  credited  in  the  text. 

THE  AUTHORS 
CORNELL  UNIVERSITY, 

Ithaca,  N.  Y. 
October  18,  1909. 


OUTLINE  AND  TABLE 
OF  CONTENTS 

PAGE 

A..  THE  SOIL  AS  A  MEDIUM  FOR  ROOT-DEVELOPMENT  1 

1.  The  rock  and  its  products 2 

I.  The  elements  of  plant-food 3 

a.  Elements  essential  to  plant-growth  (1).* 

b.  General  abundance  of  plant-food  elements  (2). 

II.  Important  soil-forming  minerals 4 

a.  Soil-forming    minerals,     their    composition    and 

properties  (3). 

b.  Relative  abundance  of  common  minerals  (4). 

III.  Important  soil-forming  rocks 9 

o.  Igneous,    Aqueous,     ^Eolian    and    Metamorphic 

rocks  (5). 
TV.  Chemical  and  physical  agencies  of  rock  decay.  ...      14 

a.  Atmosphere  (6). 

b.  Heat  and  cold  (7). 
r.  Water  (8). 

(I.  Ice — Glaciers  (9). 
e.  Plants  and  animals  (10). 
V.  Geological    classification    and    chemical    composition 

of  soils 30 

a.  Sedentary  soils  (11). 

(1)  Residual  (12). 

(2)  Cumulose  (13). 

6.  Transported  soils  (14). 

(1)  Gravity  or  Colluvial  (15) 

(2)  Water  (16). 

(a)  Marine  soils  (17). 

(b)  Lacustrine  soils  (18). 
(r)  Alluvial  soils  (19). 

(3)  Ice— Glacial  soils  (20). 

(4)  Wind— ^olian  soils  (21). 
*Number  ID  parenthesis  refers  to  section 

(xi) 


xii  OUTLINE   AND   TABLE  OF  CONTENTS 

PAGE 

VI.  Humid  and  arid  soils 64 

VII.  R6sum6  of  scheme  of  classification  and  general  char- 
acteristics of  the  groups 66 

2.  The  soil  mass.    Physical  properties  of  the  soil  and  their 

modification 68 

a.  Soil  and  subsoil  (22). 

I.  Inorganic  constituents 69 

a.  Texture  (23). 

(a)  Textural  classification  (24). 

(1)  Textural  groups  (25). 

(2)  Agricultural  classes  based  on  texture  (26). 
(6)  Some  physical  properties  of  arid  and  humid 

soils  (27). 

(c)  Some  properties  of  soil  separates  and  classes 

(28). 

(1)  Number  of  particles  (29). 

(2)  Surface  area  of  particles  (30). 

(3)  Chemical  composition  of  soil  separates  (31). 

(d)  Modification  of  soil  texture  (32). 
5.  Structure  (33). 

(a)  Some  aspects  of  soil  structure  (34). 

(1)  Ideal  arrangement  (35). 

(2)  Porosity  (36). 

(3)  Weight  (37). 

(4)  Plasticity  (38). 

(5)  Cementing  materials  (39). 

(6)  Color  (40). 

(7)  Physical  absorption  (41). 

(6)  Conditions  affecting  structure  (42). 
(c)  Means  of  modifying  structure  (43). 

(1)  Variation  in  moisture  content  (44). 

(2)  Formation  of  ice  crystals  (45). 

(3)  Tillage  (46). 

(4)  Growth  of  plant  roots  (47). 

(5)  Organic  matter  (48). 

(6)  Soluble  salts  (49). 

(7)  Animal  life  (50). 

(8)  Rainfall  (51). 


OUTLINE  AND   TABLE  OF  CONTENTS  xiii 

PAGE 

II.  Organic  constituents  of  the  soil 119 

a.  Sources,  derivation  and  forms  (52). 

b.  Chemical  composition  (53). 

c.  Amounts  present  (54). 

d.  Some  physical  properties  (55). 

(1)  Solubility  (56). 

(2)  Weight  (57). 

(3)  Absorptive  properties  (58). 

(4)  Volume  changes  (59). 

(5)  Plasticity  (60). 

e.  Effects  of  organic  matter  (61). 

(1)  Physical  effects  (62). 

(2)  Chemical  effects  (63). 

/.  Maintenance  of  organic  matter  (64). 

B.  THE  SOIL  AS  A  RESERVOIR  FOR  WATER 133 

I.  Functions  in  plant-growth 133 

II.  Amount  of  water  in  the  soil 135 

Determined  by 

a.  The  supply  (65). 

6.  Retentive  capacity  (66). 

1.  Statement  of  water-content  (67). 

2.  Forms  and  availability  (68). 

3.  Amounts  of  each  form  (69). 

(«)  Hygroscopic  water  (70). 

(b)  Capillary  water  (71). 
Determined  by 

(1)  Texture  (72). 

(2)  Structure  (73). 

(3)  Content  of  organic  matter  (74). 

(a')  Volume  of  water  held  by  different  soila 
— maximum,  minimum  and  opti- 
mum water-content  (75). 

(6')  Available  water  in  some  field  soils  (76). 

(r')  Relation  of  surface  tension  to  capil- 
larity (77). 

(c)  Gravitational  water  (78). 
c.  Amount  and  rate  of  loss  (79). 


XIV  OUTLINE   AND   TABLE  OF  CONTENTS 

PAGE 

III.  Movement  of  soil-water 165 

a.  Gravitational  movement  (80). 

b.  Capillary  or  film  movement  (81). 

1.  Principles  governing  capillary  movement  (82). 

2.  Extent,  rate  and  importance  of  capillary  move- 

ment (83). 
Determined  by 
(a)  Texture  (84). 
(6)  Dampness  of  soil  particles  (85). 

(c)  Structure  (86). 

(d)  Surface  tension  (87). 

(e)  Condition  of  surfaces  of  particles  (88). 

3.  Examples  of  amount  of  water  moved  (89). 

c.  Thermal  movement  (90). 

IV.  Control  of  soil-water 190 

a.  Means  of  increasing  water-content  of  the  soil  (91). 

1.  Decreasing  loss  (92). 

(a)  Percolation  (93). 
(6)  Evaporation  (94). 

(1)  Mulches  (95). 

(a')  Mulching  plow  land  (96). 
(6')  Fall  and  spring  plowing  (97). 

(2)  Other  surface  treatments  (98) 

2.  Increasing  the  water  capacity  (99). 

3.  Irrigation  (100). 

(a)  Factors  affecting  the  duty  of  water  (101). 
(6)  Methods  of  applying  water  (102). 

(1)  Floodings  (103). 

(2)  Furrows  (104). 

(3)  Overhead  sprays  (105). 

(4)  Sub-irrigation  (106). 

6.  Means  of  decreasing  the  water-content  of  the  soil  (107). 
1.  Drainage  by  ditches  (108). 
(a)  Effects  of  drainage  (109). 

(1)  Firms  the  soil  (110). 

(2)  Improves  the  structure  (111). 

(3)  Increases  the  available  water  (112). 


OUTLINE   AND   TABLE  OF  CONTENTS  XV 

PAQK 

(4)  Improves  the  aeration  (113). 

(5)  Raises  the  average  temperature  (114). 

(6)  Influences  the  growth  of  organisms  (115). 

(7)  Increases  the  food-supply  (116). 

(8)  Enlarges  the  root-zone  (117). 

(9)  Reduces  "heaving"  (118). 

(10)  Removes  injurious  salts  from  alkali  soils 

(119). 

(11)  Reduces  erosion  (120). 

(12)  Increases  crop-yields  and  improves  eani- 

tary  conditions  (121). 
(6)  Principles  of  drainage  (122). 

(1)  Open  or  surface  drains  (123). 

(2)  Covered  or  under-drains  (124). 

2.  Other  types  of  drainage  (125). 

3.  Surface  culture  (126). 

C.  PLANT  NUTRIENTS  OF  THE  SOIL 267 

I.  Solubility  of  the  soil  through  natural  processes 267 

II.  Solubility  of  the  soil  in  various  solvents 268 

a.  Complete  solution  of  the  soil  (127). 

b.  Digestion  with  strong  hydrochloric  acid  (128). 

1.  Interpretation   of   results   of   analysis   of  hydro- 
chloric acid  solution  (129). 

(a)  Permanent  fertility  and  manurial  needs  (130) 

(b)  Relation  of  texture  to  solubility  (131). 

(c)  Nature  of  subsoil  (132). 

(d)  Calcium  carbonate  (133). 

(e)  Estimation  of  deficiency  of  ingredients  (134). 
(/)  Conclusions  (135). 

c.  Extraction  with  dilute  organic  acids  (136). 

1.  Advantages  in  showing  manurial  needs  (137). 

2.  Usefulness  of  citric  acid  (138). 

d.  Extraction  with  aqueous  solution  of  carbon  dioxid 

(139). 

e.  Extraction  with  pure  water  (140). 

1.  Influence  of  absorption  (141). 

2.  Other  factors  (142). 


xvi  OUTLINE  AND  TABLE  OF  CONTENTS 

PAGE 

III.  Mineral  substances  absorbed  by  plants   279 

a.  Substances  found  in  ash  of  plants  (143). 

b.  Amounts  of  plant-food  material  removed  by  crops(144). 

c.  Amounts  of  plant-food  material  contained  in  soils(145). 

d.  Possible  exhaustion  of  mineral  nutrients  (146). 

IV.  Acquisition  of  nutritive  salts  by  agricultural  plants ....   286 
o.  Selective  absorption  (147). 

6.  Relation  between  root-hairs  and  soil-particles  (148). 
c.  Absorptive  power  of  different  crops  (149). 

1.  Extent  of  absorbing  system  (150). 

2.  Osmotic  activity  (151). 

3.  Cereal  crops  (152). 

4.  Grass  crops  (153). 

5.  Leguminous  crops  (154). 

6.  Root  crops  (155). 

7.  Vegetables  (156). 

8.  Fruits  (157). 

V.  Absorption  by  the  soil  of  substances  in  solution 297 

a.  Substitution  of  bases  (158). 

6.  Time  required  for  absorption  (159). 

c.  Insolubility  of  certain  absorbed  substances  (160). 

d.  Influence  of  size  of  particles  (161). 

e.  Causes  of  absorption  (162). 

1.  Zeolites  (163). 

2.  Other  absorbents  (164). 
/.  Adsorption  (165). 

g.  Occlusion  (166). 

h.  Adsorption  as  related  to  drainage  (167). 

1.  Substances  usually  carried  in  drainage  water  (168). 

2.  Drainage  records  at  Rothamsted  (169). 

t .  Relation  of  absorptive  capacity  to  productiveness  ( 1 70) . 

VI.  Alkali  soils 307 

a.  Composition  of  alkali  salts  (171). 

6.  White  and  black  alkali  (174). 
c.  Effect  of  alkali  on  crops  (173). 

1.  Direct  effect  (174). 

2.  Indirect  effect  (175). 


OUTLINE   AND   TABLE  OF  CONTENTS  xvii 

PAGE 

3.  Effect  upon  different  crops  (176). 

4.  Other  conditions  influencing  the  action  of  alkali 

(177). 
d.  Reclamation  of  alkali  land  (178). 

1.  Irrigation  and  alkali  (179). 

2.  Under-drainage  (180). 

3.  Correction  of  black  alkali  (181). 

4.  Retarding  evaporation  (182). 

6.  Cropping  with  tolerant  plants  (183). 

6.  Other  methods  (184). 

7.  Alkali  spots,  (185). 

VII.  Manures 319 

a.  Early  ideas  of  the  function  of  manures  (186). 

b.  Development  of  the  idea  of  the  nutrient  function  of 

manures  (187). 

c.  Classes  of  manures  (188). 

1.  Commercial  fertilizers  (189). 

(a)  Function  of  commercial  fertilizers  (190). 
(6)  Fertilizer  constituents  (191). 

(c)  Fertilizers  used  for  their  nitrogen  (192). 

(1)  Sodium  nitrate  (193). 

(2)  Ammonium  sulfate  (194). 

(3)  Calcium  cyanamid  (195). 

(4)  Calcium  nitrate  (196). 

(5)  Organic  nitrogen  in  fertilizers  (197). 

(d)  Fertilizers  used  for  their  phosphorus  (198). 

(1)  Bone  phosphate  (199). 

(2)  Mineral  phosphates  (200). 

(3)  Superphosphate  fertilizers  (201). 

(4)  Reverted  phosphoric  acid  (202). 

(5)  Double  superphosphates  (203). 

(6)  Relative    availability    of    phosphate    fer- 

tilizers (204). 

(e)  Fertilizers  used  for  their  potassium  (205). 

(1)  Stassfurt  salts  (206). 

(2)  Wood-ashes  (207). 

(3)  Insoluble  potassium  fertilizers  (208). 


XViii  OUTLINE  AND   TABLE   OF  CONTENTS 

i 

2.  Fertilizer  practice  (209). 

(a)  Brands  of  fertilizers  (210). 
(6)  Fertilizer  inspection  (211). 

(c)  Trade  values  of  fertilizers  (212). 

(d)  Computation  of  the  commercial  value  of  a 

fertilizer  (213). 

(e)  Mixing  fertilizers  on  the  farm  (214). 
(/)  Methods  of  applying  fertilizers  (215). 

3.  Soil  amendments  (216). 

(a)  Salts  of  calcium  (217). 

(1)  Effect  on  tilth  (218). 

(2)  Liberation  of  plant-food  materials  (219). 

(3)  Effect    on    toxic    substances    and    plant 

diseases  (220). 

(4)  Forms  of  calcium  (221). 
(a')  Caustic  lime  (222). 

(6')  Carbonate  of  lime  (223). 
(c')  Sulfate  of  lime  (224). 

(b)  Common  salt  (225). 

(c)  Muck  (226). 

4.  Factors  affecting  the  efficiency  of  fertilizers  (227). 

(a)  Soil-moisture  content  (228). 
(6)  Soil-acidity  (229). 

(c)  Organic  matter  (230). 

(d)  Structure  or  tilth  of  the  soil  (231). 

(e)  Cumulative  need  for  fertilizer  (232) 

5.  Farm  manures  (233). 

(a)  Solid  excreta  (234). 
(fe)  Urine  (235). 

(c)  Litter  (236). 

(d)  Manures  produced  by  different  animals  (237). 

(1)  Horse  manure  (238). 

(2)  Cow  manure  (239). 

(3)  Swine  manure  (240). 

(4)  Sheep  manure  (241). 

(5)  Relative  values  of  animal  manures  (242). 

(6)  Poultry  manure  (243). 


OUTLINE   AND  TABLE  OF  CONTENTS  XIX 

PAGE 

(e)  Factors  affecting  the  values  of  manures  (244). 

(1)  Age  of  animal  (245). 

(2)  Food  of  animal  (246). 

(3)  Use  of  animal  (247). 

(/)  Deterioration  of  farm  manure  (248). 

(1)  Fermentations  (249). 

(2)  Leaching  (250). 

(3)  Methods  of  handling  (251). 
(h)  Place  in  rotation  (252). 
(t)  Functions  (253). 
6.  Green  manures  (254). 

(a)  Leguminous  crops  (255). 

(b)  Cereal  crops  (256). 

D.  ORGANISMS  IN  THE  SOIL 388 

I.  Macro-organisms  of  the  soil 388 

a.  Rodents  (257). 

b.  Worms  (258). 

c.  Insects  (259). 

d.  Large  fungi  (260). 

e.  Plant-roots  (261). 

II.  Micro-organisms  of  the  soil 391 

a.  Plant  micro-organisms  (262). 

1.  Plant  micro-organisms  injurious  to  higher  plants 

(263). 

2.  Plant  micro-organisms  not  injurious  to  higher(264). 

3.  Bacteria  (265). 

(a)  Distribution  (266). 

(b)  Numbers  (267). 

(c)  Conditions  affecting  growth  (268). 

(1)  Oxygen  (269). 

(2)  Moisture  (270). 

(3)  Temperature  (271). 

(4)  Organic  matter  (272). 

(5)  Soil  acidity  (273). 

(d)  Functions  of  soil  bacteria  (274). 

(1)  Decomposition  of  mineral  matter   (275). 


XX  OUTLINE  AND   TABLE  OF  CONTENTS 

PAGE 

(2)  Decomposition  of  non-nitrogenous  organic 

matter  (276). 

(3)  Decomposition    of    nitrogenous    organic 

matter  (277). 

(a')  Decay  and  putrefaction  (278). 
(6')  Ammonification  (279). 
(c')  Nitrification  (280). 

(!')  Effect  of  organic  matter  on  nitri- 
fication (281). 

(2')  Effect  of  soil-aeration  on  nitrifica- 
tion (282). 

(3')  Effect  of  sod  on  nitrification  (283). 
(4')  Depth  at  which  nitrification  takes 

place  (284). 
(5')  Loss  of  nitrates  from  the  soil  (285). 

(4)  Denitrification  (286). 

(5)  Nitrogen  fixation  through  symbiosis  with 

higher  plants  (287). 
(a')  Relation   of   bacteria   to   nodules   on 

roots  (288). 

(&')  Transfer  of  nitrogen  to  the  plant  (289). 
(c')  Soil-inoculation  for  legumes  (290). 

(6)  Nitrogen  fixation  without  symbiosis  with 

higher  plants  (291). 
(a')  Nitrogen-fixing  organisms  (292). 
(6')  Mixed     cultures     of     nitrogen-fixing 

organisms  (293). 
(O  Nitrogen-fixation   and   denitrification 

antagonistic  (294). 

E.  THE  SoiL-Am   432 

I.  Factors  determining  yolume   432 

a.  Texture  (295). 
6.  Structure  (296). 

c.  Organic  matter  (297). 

d.  Moisture  content  (298). 

II.  Composition  of  soil-air    434 

a.  Analysis  of  soil-air  (299). 

b.  Production  of  carbon  dioxid  as  affecting  composition 

(300). 


OUTLINE  AND  TABLE  OF  CONTENTS  XXI 

PAGE 

c.  Escape  of  carbon  dioxid  as  affecting  composition  (301). 

d.  Effect  of  roots  upon  composition  (302). 

III.  Functions  of  the  soil-air 437 

a.  Oxygen  (303). 

b.  Carbon  dioxid  (304). 

IV.  Movement  of  soil-air 439 

a.  Diffusion  of  gases  (305). 

b.  Movement  of  water  (306). 

c.  Changes  in  atmospheric  pressure  (307). 

d.  Changes  in  temperature  (308). 

e.  Suction  produced  by  wind  (309). 

V.  Methods  for  modifying  the  volume  and  movement  of  soil-  443 
air 

a.  Tillage  (310). 

b.  Manures  (311). 

c.  Under-drainage  (312). 

d.  Irrigation  (313). 

e.  Cropping  (314). 

?.  HEAT  OF  THE  SOIL   448 

I.  Function  of  the  heat  of  the  soil  in  its  relation  to  plant- 
growth    448 

a.  Biological  (315). 

1.  Germination  (316). 

2.  Growth  and  vegetation  (317). 

3.  Activity  of  the  soil-organisms  (318). 

b.  Chemical  changes  (319). 

c.  Physical  changes  (320). 

II.  Sources  of  the  heat  of  the  soil 451 

a.  Solar  radiation  (321). 
6.  Conduction  (322). 
c.  Organic  decay  (323). 

III.  Temperature  of  the  soil 463 

a.  Heat  supply  (324). 

6.  Specific  gravity  and  specific  heat  (325). 

c.  Color  of  the  soil  (326). 

d.  Slope  of  the  soil  (327). 


xxii  OUTLINE   AND   TABLE  OF  CONTENTS 

PAGE 

e.  Conductivity  (328). 
/.  Circulation  of  air  (329). 
g.  Water  content  (330). 

IV.  Means  of  modifying  the  soil  temperature 463 

G.  EXTERNAL  FACTORS  IN  SOIL-MANAGEMENT 465 

I.  Means  of  modifying  the  soil 4.65 

a.  Summary  of  practices  (331). 
II.  Tillage 466 

a.  Objects  of  tillage  (332). 

b.  Implements  of  tillage  (333). 

1.  Effect  on  the  soil  (334). 

2.  Mode  of  action  (335). 

(a)  Plows  (336). 

(1)  Pulverization  (337). 

(2)  Covering  rubbish  (338). 
(6)  Cultivators  (339). 

(1)  Cultivators  proper  (340). 

(2)  Leveler  and  harrow  type  of  cultivator(341). 

(3)  Seeder  cultivators  (342). 
(e)  Packers  and  crushers  (343). 

(1)  Rollers  (344). 

(2)  Clod  crushers  (345). 

III.  Other  phases  of  tillage  operations 489 

a.  Weeds  in  their  relation  to  crop-production  (346). 

1.  Objectionable  qualities  of  weeds  (347). 

2.  Control  of  weeds  (348). 

b.  Erosion  (349). 

1.  By  water  (350). 

2.  By  wind  (351). 

IV.  Adaptation  of  crops  to  soil 497 

a.  Philosophy  of  crop-adaptation  (352). 

b.  Factors  in  crop-adaptation  (353). 

1.  Physiological    requirements   of   the   plant    (354). 

2.  Requirements  for  growth  supplied  by  the  soil  (355). 


OUTLINE   AND   TABLE  OF  CONTENTS  xxiii 

PAOB 

V.  Relation  of  soil-productiveness  to  crop-rotations 503 

a.  Principles  underlying  crop-rotation  (356). 

1.  Nutrients  removed  from  the  soil  by  different  crops 

(357). 

2.  Root  systems  of  different  crops  (358). 

3.  Some  crops  or  crop  treatments  prepare  food  for 

other  crops  (359). 

4.  Crops  differ  in  their  effect  upon  soil  structure  (360). 

5.  Certain  crops  check  certain  weeds  (361). 

6.  Plant  diseases  and  insects  checked  by  removal  of 

hosts  (362). 

7.  Loss  of  plant  food  from  unused  soil  (363). 

8.  Accumulation  of  toxic  substances  (364). 


CLASSIFIED    LIST    OF 
ILLUSTRATIONS 

PAOE 

1.  "Plowing" Frontispiece 

2.  Map  of  United  States,  normal  annual  precipitation.   Fig.  41  ....  137 

3.  Map  of  western  United  States,   irrigated    and    irrigable  land. 

Fig.   72 227 

4.  Map  of  United  States,  showing  relative  use  of  fertilizers.    Fig. 

108 323 

5.  Map  of  United  States,  sunshine  received  in    different  sections. 

Fig.  121 452 

6.  Rock  section.    Granite.    Fig.  1   8 

7.  Rock  section.    Diorite.    Fig.  2 10 

8.  Rock  section.   Basalt.   Fig.  3 11 

9.  Rock  section.    Fossiliferous  limestone.    Fig.  4 12 

10.  Rock  section.    Chert.    Fig.  5 13 

11.  Rock  section.    Sandstone.    Fig.  6 13 

12.  Photo-micrograph.    Fine  sand.    Fig.  17 70 

13.  Photo-micrograph.    Silt.    Fig.  18 71 

14.  Influence  of  water  film  on  soil 'granulation.    Fig.  30 105 

15.  Weathered  Laramie  sandstone.    Fig.  7 15 

16.  Types  of  weathering.    Fig.  8 19 

17.  Weathered  limestone  in  quarry.    Fig.  9 22 

18.  "Pot-holes"  in  shale  rock.    Fig.  10 25 

19.  Lichen  on  granite  rock.    Fig.  11 28 

20.  Tree  roots  spliting  boulder.    Fig.  12 29 

21.  Section  of  residual  soil  from  limestone.    Fig.  13 38 

22.  Section  of  muck  underlain  by  marl.    Fig.  14 42 

23.  Section  of  sedimentary  soil.    Fig.  15 48 

24.  Section  of  glacial  soil.    Fig.  16 58 

25.  Proportion  of  separates  in  sandy  soil.    Fig.  21 75 

26.  Proportion  of  separates  in  silt  soil.    Fig.  22 75 

27.  Proportion  of  separates  in  clay  soil.    Fig.  23 76 

28.  Undesirable  soil  structure.    Fig.  27 90 

29.  Ideal  soil  structure.    Fig.  28 91 

30.  Excessive  checking  of  clay  soil.    Fig.  29 99 

31.  Ice  crystals  in  field  soil.    Fig.  31 108 

32.  Honey-comb  ice  crystals.    Fig.  32 109 

(xxv) 


XXvi  CLASSIFIED  LIST  OF  ILLUSTRATIONS 

PAGE 

33.  Ice  crystals  and  soil  granulation.    Fig.  33 110 

34.  Clay  soil  plowed  wet.    Fig.  35 112 

35.  Soil  in  good  tilth.    Fig.  38 126 

36.  A  mulch  of  stone.   Fig.  63 201 

37.  Example  of  clean  cultivation.    Fig.  65 206 

38.  Flume  for  measuring  miner's  inches.    Fig.  73 227 

39.  Canvas  dam.   Fig.  74 232 

40.  Poorly  drained  clay  land.   Fig.  76 238 

41.  Section  of  tile  drain  in  clay  soil.   Fig.  77 241 

42.  Soil  granulated  by  drainage.   Fig.  79 246 

43.  System  of  surface  drains.   Fig.  80 249 

44.  Construction  of  ditch  for  tile  drain.    Fig.  81 251 

45.  Laying  tile  by  use  of  tile  hook.    Fig.  82 252 

46.  Laying  tile  by  hand.   Fig.  83 253 

47.  Types  of  drain  tile.   Fig.  88 257 

48.  Ditching  machine  in  operation.   Fig.  91 260 

49.  Ditch  cut  by  machine.    Fig.  92 261 

50.  Poorly  constructed  outlet  of  drain.    Fig.  93 262 

51.  Result  of  poorly  constructed  outlet.    Fig.  94 264 

52.  Well-constructed  outlet.    Fig.  95 265 

53.  Filling  ditch  by  use  of  team.   Fig.  96 266 

54.  Waste  of  manure  by  leaching.   Fig.  101 302 

55.  Alkali  spot.   Fig.  102 308 

56.  Bromus  inermis  on  alkali  soil.    Fig.  104 317 

57.  Wates  of  manure.   Fig.  106 364 

58.  Manure  piled  in  the  field.    Fig.  107 382 

59.  Alfalfa  root  tubercles.   Fig.  114 424 

60.  Heavy  sod  freshly  broken.    Fig.  120 445 

61.  Erosion  on  gravelly  hillside.    Fig.  150 491 

62.  Terraces  used  in  southern  farming.    Fig.  151 492 

63.  Side-hill  ditches  to  prevent  erosion.   Fig.  152 493 

64.  Plant  roots  and  erosion.    Fig.  153 495 

65.  Celery  and  lettuce  on  muck  soil.   Fig.  154 498 

66.  Farm  scene  on  light  sand  soil.    Fig.  155 500 

67.  Farm  scene  on  limestone  loam  soil.    Fig.  156 502 

68.  Influence  of  crop  rotation  on  growth  of  corn.    Fig.  157 507 

69.  Diagram.   Relative  size  of  textural  groups.   Fig.  19 72 

70.  Diagram.   Ideal  arrangement  of  soil  particles.   Fig.  26 89 

71.  Diagram.    Forms  and  proportion  of  soil  water.    Fig.  43 141 

72.  Diagram.    Distribution  of  soil  water.    Fig.  45 147 

73.  Diagram.    Adjustment  of  capillary  soil  water.    Fig.  52 171 

74.  Diagram.    Relation  of  root-hairs  to  soil  water.    Fig.  53 174 

75.  Diagram.    Structure  of  mulched  and  unmulched  soil.    Fig.  62.  .  199 

76.  Diagram.   Water  table  in  tile-drained  land.   Fig.  78 245 


CLASSIFIED  LIST  OF  ILLUSTRATIONS  xxvii 

PAGE 

77.  Diagram.  Gridiron  system  of  arranging  drains.   Fig.  84 254 

78.  Diagram.  System  of  arranging  drains.    Fig.  85 255 

79.  Diagram.  System  of  arranging  drains.    Fig.  86 255 

80.  Diagram.  Natural  system  of  arranging  drains.   Fig.  87 256 

81.  Diagram.  Sectional  view  of  ditching  machine.   Fig.  90 259 

82.  Diagram.  Relation  of  root-hairs  to  soil  particles.   Fig.  99.  ...  287 

83.  Diagram.  Effect  of  deep  and  shallow  tillage  on  roots.  Fig.  100 .  294 

84.  Diagram.  Effect  of  alkali  salts  on  plant  cells.    Fig.  103 312 

85.  Diagram.  Nematodes  entering  a  root.    Fig.  108 392 

86.  Diagram.  Types  of  soil  bacteria.   Fig.  109 395 

87.  Diagram.  Influence  of   surface    slope    on   sunshine    received. 

Fig.  125 458 

88.  Curves.  Relative  size  of  textural  groups.   Fig.  20 74 

89.  Curves.  Average  analysis  of  common  classes  of  soil.    Fig.  24  .  78 

90.  Curves.  Relation  of  texture  to  crop  adapation.    Fig.. 25 79 

91.  Curves.  Relation  of  texture  to  water  capacity.    Fig.  44 145 

92.  Curves.  Distribution  of  water  in  columns  of  soil.    Fig.  46.  ...  148 

93.  Curves.  Relation  of  texture  to  capillary  water  capacity.  Fig.  47.  150 

94.  Curves.  Relation  of  structure  to  water  capacity.    Fig.  48  ....  152 

95.  Curves.  Water  capacity  of  sandy  soil  in  field.    Fig.  49 156 

96.  Curves.  Water  capacity  of  clay  soil  in  field.    Fig.  50 157 

97.  Curves.  Water  capacity  of  silt  soil  in  field.    Fig.  51 157 

98.  Curves.  Relation  of  capillary  rise  to  texture.    Fig.  54 175 

99.  Curves.  Relation  of  capillary  rise  to  texture.    Fig.  55 177 

100.  Curves.  Relation  of  capillary  rise  to  texture.    Fig.  56 179 

101.  Curves.  Relation  of  capillary  rise  to  texture.   Fig.  57 179 

102.  Curves.  Lateral  capillary  movement.    Fig.  58 184 

103.  Curves.  Annual  precipitation  and  percolation, England.  Fig.  61.   193 

104.  Curves.  Relation  of  evaporation  to  mulch  formation.    Fig.  64.    204 

105.  Curves.  Relation  of  soil  moisture  to  yield  of  dry  matter.  Effect 

of  weeds.   Fig.  66 209 

106.  Curves.    Influence  of  cloth  tent  on  soil  moisture.    Fig.  68  ....    215 

107.  Curves.    Relation  of  soil  condition  to  the  formation  of  nitrates. 

Fig.  112 414 

108.  Curves.    Daily  range  of  soil  temperature.    Fig.  122 454 

109.  Curves.    Mean  annual  range  of  air  and  soil  temperature.    Ne- 

braska.   Fig.    123 455 

110.  Curves.    Effect  of  color  on  soil  temperature.    Fig.  124 457 

111.  Moldboard  plow  with  shares.    Fig.  34 Ill 

112.  Middlebreaker  plow.    Fig.  75 236 

113.  Hillside  plow.    Fig.  118 441 

114.  Plow  with  parts  named.    Fig.  127 466 

1 15.  Heel-plate  of  plow.    Fig.  128 467 

116.  Sulky  moldboard  plow.    Fig.  129 468 


XXviii          CLASSIFIED  LIST  OF  ILLUSTRATIONS 

PAGE 

117.  Sulky  disc  plow.   Fig.  135 476 

118.  Six  gang  plow.   Fig.  132 473 

119.  Subsoil  plow.   Fig.  69 218 

120.  Subsoil  plow.   Fig.  70 219 

121.  One-horse  toothed  cultivator.   Fig.  60 191 

122.  Weeder.   Fig.  59 187 

123.  Large-shovel  cultivator.    Fig.  Ill 407 

124.  Small-shovel  cultivator.   Fig.  113 419 

125.  Spring-toothed  cultivator.    Fig.  114 402 

126.  Blade  cultivator.   Fig.  115 433 

127.  Disc  cultivator.   Fig.  116 436 

128.  Hand  cultivator.   Fig.  117 438 

129.  "Sweep"  cultivator.    Fig.  137 479 

130.  Hand  tillage  implements.    Fig.  98 281 

131.  Spring-toothed  harrow.    Fig.  36 114 

132.  Spike-toothed  harrow.    Fig.  42 140 

133.  Solid-disc  harrow.   Fig.  39 130 

134.  Meeker  harrow.   Fig.  140 487 

135.  Meeker  harrow.    Near  view.    Fig.  37 118 

136.  Cut-out-disc  harrow.   Fig.  97 276 

137.  Spading-disc  harrow.    Fig.  126 463 

138.  Extension  disc  harrow.    Fig.  136 477 

139.  Acme  harrow.    Fig.  119 444 

140.  Clod  crusher.   Fig.  71 220 

141.  Scotch  chain  harrow.    Fig.  146 486 

142.  Solid  or  barrel  roller.    Fig.    40 135 

143.  Bar  roller.    Fig.    147 487 

144.  "Float"  or  plank  smoother.    Fig.  149 489 

145.  Campbell  sub-surface  packer.    Fig.  67 212 

146.  Broadcast  seeder.    Fig.  138 480 

147.  Grain  drill.    Fig.  144 485 

148..  Sulky  lister.    Fig.    130 470 

149.  One-horse  grain  drill.    Fig.  139 481 

150.  Stubble  digger.    Fig.  143 484 

151.  Garden  seeder.   Fig.  133 474 

152.  Berry  hoe.    Fig.  134 475 

153.  Beet  loosener.    Fig.  141 482 

154.  Cotton-and-corn  planter.   Fig.  142 483 

155.  Corn  planter.   Fig.  145 486 

156.  Potato  digger.   Fig.  148. . . . , 488 

157.  Hand  drainage  tools.    Fig.  89 258 

158.  Types  of  coulters.   Fig.  131 471 


INTRODUCTION 

By  L.  H.  BAILEY 

The  exposed  surface  of  the  crust  of  the  earth  tends 
always  to  pass  into  a  loose  and  disintegrated  layer. 
In  this  layer  many  organisms  live,  and  out  of  it  many  of 
them  derive  an  essential  part  of  their  nourishment. 
The  organisms  die  and  their  remains  return  to  the  place 
whence  they  came.  In  every  successive  epoch  of  the 
earth's  history,  this  layer  has  tended  to  become  more 
differentiated  and  complex  in  each  epoch  supporting 
a  higher  type  of  plant,  and  in  each  succeeding  age  main- 
taining a  more  advanced  kind  of  activity.  Thus  the  soil 
has  been  formed,  and  the  evolution  of  it  and  of  the  plant 
tribes  that  grow  out  of  it  have  been  reciprocal,  one  con- 
tributing to  the  other.  If  the  soil  is  essential  to  the 
growing  of  plants,  so  have  the  plants  been  essential  to 
the  formation  of  soil. 

This  marvelously  thin  layer  of  a  few  inches  or  a  very 
few  feet  that  the  farmer  knows  as  "the  soil,"  supports 
all  plants  and  all  men,  and  makes  it  possible  for  the  globe 
to  sustain  a  highly  developed  life.  Beyond  all  calculation 
and  all  comprehension  are  the  powers  and  the  mysteries 
of  this  soft  outer  covering  of  the  earth.  We  do  not  know 

(xxix) 


XXX  INTRODUCTION 

that  any  vital  forces  pulsate  from  the  great  interior  bulk 
of  the  earth.  For  all  we  know,  the  stupendous  mass  of 
materials  of  which  the  planet  is  composed  is  wholly 
dead;  and  only  on  the  veriest  surface  does  any  nerve  of 
life  quicken  it  into  a  living  sphere.  And  yet,  from  this 
attenuated  layer  have  come  numberless  generations 
of  giants  of  forests  and  of  beasts,  perhaps  greater  in 
their  combined  bulk  than  all  the  soil  from  which  they 
have  come;  and  back  into  this  soil  they  go,  until  the  great 
life  principle  catches  up  their  disorganized  units  and 
builds  them  again  into  beings  as  complex  as  themselves. 

The  general  evolution  of  this  soil  is  toward  greater 
powers;  and  yet,  so  nicely  balanced  are  these  powers  that 
within  his  lifetime  a  man  may  ruin  any  part  of  it  that 
society  allows  him  to  hold;  and  in  despair  he  throws  it 
back  to  nature  to  reinvigorate  and  to  heal.  We  are  ac- 
customed to  think  of  the  power  of  man  in  gaining  domin- 
ion over  the  forces  of  nature, — he  bends  to  his  use  the 
expansive  powers  of  steam,  the  energy  of  electric  cur- 
rents, and  he  ranges  through  space  in  the  light  that  he 
concentrates  in  his  telescope;  but  while  he  is  doing  all 
this  he  sets  at  naught  the  powers  in  the  soil  beneath  his 
feet,  wastes  them,  and  deprives  himself  of  vast  sources 
of  energy.  Man  will  never  gain  dominion  until  he  learns 
from  nature  how  to  maintain  the  augmenting  powers 
of  the  disintegrating  crust  of  the  earth. 

There  are  three  great  kinds  of  natural  resources, — ' 


INTRODUCTION  XXxi 

the  earth  itself,  the  atmosphere  that  envelopes  it  and  which 
maybe  considered  an  outer  layer  of  it,  and  the  sunshine. 
From  these  three,  and  all  the  materials  and  forces  that 
are  in  them  contained,  we  derive  the  conditions  of  our 
existence  and  express  our  outlook  to  destiny.  We  can 
do  little  to  control  or  modify  the  atmosphere  or  the 
sunlight;  but  the  surface  of  the  earth  is  ours  to  do  with 
it  much  as  we  will.  It  is  the  one  great  resource  over  which 
we  have  dominion.  Within  this  crust  are  great  stores  of 
minerals  and  of  metals  and  of  other  materials  that  we 
can  use  for  our  comfort;  these  materials  we  can  save 
and  we  may  use  them  with  economy,  but  we  cannot 
cause  them  to  increase.  But  the  soil  may  be  made  better 
as  well  as  worse,  more  as  well  as  less;  and  to  save  the 
producing  powers  of  it  is  far  and  away  the  most  import- 
ant consideration  in  the  conservation  of  natural  resources. 
The  man  who  owns  and  tills  the  soil,  therefore,  owes 
an  obligation  to  his  fellowmen  for  the  use  that  he  makes 
of  his  land;  and  his  fellowmen  owe  an  equal  obligation  to 
him  to  see  that  his  lot  in  society  is  such  that  he  will 
not  be  obliged  to  rob  the  earth  in  order  to  maintain  his 
life.  The  natural  resources  of  the  earth  are  the  heritage 
and  the  property  of  every  one  and  all  of  us.  We  shall 
reach  the  time  when  we  shall  not  allow  a  man  to  till  the 
earth  unless  he  is  able  to  leave  it  at  least  as  fertile  as  he 
found  it.  A  man  has  no  moral  right  to  skin  the  earth, 
unless  he  is  forced  to  do  it  in  sheer  self-defence  and  to 


XXxii  INTRODUCTION 

enable  him  to  live  in  some  epoch  of  an  unequally  devel- 
oped society;  and  if  there  are  or  have  been  such  social 
epochs,  then  is  society  itself  directly  responsible  for  the 
waste  of  the  common  heritage. 

On  every  side,  therefore,  it  is  important  that  we  study 
the  soil.  Beyond  all  mere  technical  agricultural  practice, 
the  principles  of  soil  management  must  be  compre- 
hended and  taught.  There  is  no  good  sociology  that  does 
not  recognize  this  fact. 

We  tend  always  to  discuss  great  subjects  from  one 
point  of  view.  So  has  the  soil  usually  been  treated  from 
the  chemical  point  of  view,  from  the  geological,  from  the 
agricultural.  In  this  book,  the  authors  have  attempted 
to  discuss  the  soil  in  all  its  relations  to  plant  production, 
developing  the  inter-dependence  of  geological,  chemical, 
bacteriological,  physical  and  industrial  relationships  in 
such  a  way  as  to  give  the  student  a  grasp,  albeit  a  brief 
one,  of  the  entire  subject  in  its  many  bearings.  In  its 
treatment,  the  book  considers,  first,  the  soil  as  a  medium 
for  root  development ;  second,  as  a  reservoir  for  water ; 
third,  as  a  source  of  nutrients;  fourth,  as  a  realm  of 
organisms;  fifth,  in  its  relation  to  air;  sixth,  its  relation 
to  heat;  and  the  relation  of  man  to  the  soil  follows  as  a 
consequence  and  conclusion. 

The  past  few  years  constitute  a  period  of  great  activity 
in  the  study  of  the  soil,  so  much  so  that  many  of  our  most 
established  opinions  have  been  challenged.  Perhaps  it 


INTRODUCTION  XXxiH 

is  yet  too  early  to  rationalize  all  the  new  discussions 
into  a  clear  course  of  practice,  but  we  are  surely  getting 
nearer  to  the  fundamental  problems,  and  we  shall  evolve 
a  better  system  of  agricultural  procedure.  The  stimula- 
tion of  inquiry  and  imagination  cannot  fail  to  produce 
great  results. 

So  am  I  glad  of  every  new  effort  that  puts  men  ration- 
ally on  their  feet  on  the  soil.  It  will  be  a  great  thing  when 
the  soil  is  known  in  schools.  I  wait  for  good  politics  and 
good  institutions  to  grow  out  of  the  soil.  I  wait  for  the 
time,  also,  when  we  shall  have  good  poetry  and  good 
artistic  literature  developing  from  subjects  associated 
with  the  soil;  for  we  want  good  literature  to  appeal  to 
all  men. 


THE   PRINCIPLES   OF  SOIL 
MANAGEMENT 


A.    THE  SOIL  AS  A  MEDIUM  FOR  ROOT 
DEVELOPMENT 

The  soil  is  a  medium  for  the  development  of  plants. 
In  the  main,  the  plants  which  are  of  agricultural  impor- 
tance are  differentiated  into  root  and  top,  and  the 
former  penetrates  the  soil  in  order  to  obtain  food  and 
moisture,  and  to  afford  a  firm  support  for  the  aerial 
portion.  Every  plant  has  definite  requirements  for 
its  best  development.  The  character  of  the  mature 
plant  is  the  result  of  two  sets  of  forces.  The  first  of 
these  is  the  inherent  capacity  of  the  seed  to  develop 
and  produce  a  normal  individual  of  its  kind.  The  second 
set  of  forces  constitute  the  environment  in  which  the 
plant  grows,  and  of  which  the  soil  is  one  part,  the  other 
component  being  climate.  Every  plant  is  an  expression 
of  the  combination  and  interaction  of  these  three 
groups  of  forces — the  seed,  the  climate,  and  the  soil. 

The  external  factors  in  plant  growth  may  be  further 
differentiated  into  the  following:  (1)  Food,  (2)  moisture, 
(3)  heat,  (4)  light,  (5)  air,  (0)  mechanical  support, 
and  (7)  freedom  from  biological  enemies,  such  as  fungous 
disease  and  animal  attack.  With  the  exception  of  light, 
every  one  of  these  factors  is  partially  or  wholly  deter- 

A  (1) 


2  THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

mined  by  the  character  and  condition  of  the  soil.  It 
is  the  source  of  the  majority  of  the  nutritive  elements, 
it  contains  the  w,ater  necessary  for  the  plant  and  in 
which  is  carried  its  food,  it  holds  air  in  its  pores,  and 
it  absorbs  and  transmits  the  necessary  heat.  Enemies 
of  one  plant  may  or  may  not  be  present;  but,  if  present, 
they  may  exercise  a  controlling  influence.  All  the  parts 
of  the  soil  mechanism — for  such  it  must  be  considered- 
are  closely  related  to  each  of  these  essential  factors, 
and  it  is  from  this  point  of  view  of  the  growing  plant 
that  the  following  treatment  is  developed. 

The  characteristics  of  the  soil  may  be  viewed  from 
both  the  origin  of  the  material  and  its  properties.  The 
first  of  these  may  be  termed  "The  Rock  and  Its  Prod- 
uct," and,  second, — in  so  far  as  they  pertain  to  physical 
properties, — "The  Soil  Mass.'" 


1.  THE  ROCK  AND  ITS  PRODUCTS 

Since  all  soil  material  forms  a  part  of  the  structure 
of  the  earth,  its  origin  and  derivation  constitute  a  part 
of  the  field  of  geology.  The  following  discussion  of 
the  rock  and  its  products  deals  primarily  with  these 
facts  and  processes.  But  the  discussion  is  not  taken 
up  because  of  its  geological  interest,  great  as  that  is, 
but  because  of  the  fundamental  connection  these 
have  to  the  physical,  chemical  and  biological  proper- 
ties of  the  soil  which  determine  its  ability  to  grow  plants. 
The  kinds  of  minerals  and  rocks  in  which  the  essential 
elements  of  plant-food  originally  occur,  and  the  changes 


ELEMENTS   OF   PLANT-FOOD  3 

which  they  may  have  undergone  in  their  transition  to 
the  present  combinations  in  the  soil,  as  well  as  the  fact 
that  the  physical  properties  of  the  soil  are  primarily 
determined  by  its  derivation,  render  their  study  of 
fundamental  concern  in  order  to  understand  the  soil 
as  a  medium  for  plant-growth.  The  classification  and 
detailed  study  of  the  soil  is  inseparably  linked  with  its 
derivation,  because  determined  by  it.  On  one  side,  it 
supplies  certain  elements  of  food  whose  relative  abun- 
dance is  determined  by  their  distribution  in  the  original 
rocks  and  their  concentration  or  dissipation  through 
geological  changes,  and,  on  the  other  side,  it  affords 
the  physical  medium  for  the  development  of  the  plant. 

I.     THE    ELEMENTS    OF    PLANT-FOOD 

The  plant  must  have  certain  food  elements  for  its 
growth  and  development.  These  elements  are  affected 
by  the  changes  to  which  the  rock  is  subjected,  and  in 
the  end  will  reflect  the  character  of  these  changes. 

1.  Elements  essential  to  plant-growth. — The  essen- 
tial elements  of  plant-food  are  ten  in  number,  to  which 
may  be  added  three  others  which  seem  to  be  useful 
under  certain  conditions.  The  essential  elements  may  be 
divided  into  two  groups,  on  the  basis  of  their  origin: 
(1)  The  elements  derived  entirely  and  only  from  the 
solid  portion  of  the  soil.  These  are  calcium,  magnesium, 
potassium,  phosphorus,  iron  and  sulfur.  (2)  The  ele- 
ments derived  either  directly  or  indirectly  from  air 
and  water.  These  are  carbon,  hydrogen,  oxygen  and 
nitrogen. 


4  THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

2.  General  abundance  of  the  plant-food  elements.— 

Having  now  in  mind  the  essential  food  elements  it  is 
of  interest  to  know  their  general  abundance  in  the  earth's 
crust.  The  following  table  is  given  by  Clark: 

Oxygen 47.02  Phosphorus 0.09 

Silicon 28.06  Manganese     07 

Aluminum 8.16  Sulfur 07 

Iron 4.64  Barium 05 

Calcium 3.50  Strontium    02 

Magnesium 2.62  Chromium 01 

Sodium 2.63  Nickel 01 

Potassium 2.32  Lithium 01 

Titanium 41  Chlorin 01 

Hydrogen 17  Fluorine 01 

Carbon 12 

100.00 

The  first  eight  elements  form  98.8  per  cent  of  the 
earth's  crust.  In  this  list  are  found  all  of  the  food  ele- 
ments except  nitrogen,  which  forms  four-fifths  of  the 
atmosphere.  All  of  the  food  elements  except  nitrogen 
appear  among  the  first  thirteen,  and  in  amounts  of  not 
less  than  .07  per  cent.  This  gives  assurance  that  none 
of  the  food  elements  are  rare.  It  will  appear  later  that 
they  are  all  very  generally  distributed.  The  ultimate 
source  of  the  elements  of  the  first  or  so-called  incom- 
bustible groups  is  the  minerals  of  the  earth's  crust. 

II.     IMPORTANT    SOIL-FORMING    MINERALS 

Minerals  are  the  units  of  which  soils  and  rocks  are 
primarily  composed.  A  mineral  is  a  compound  occurring 
in  nature  having  approximately  a  definite  chemical 


SOIL-FORMING   MINERALS  5 

composition,  usually  a  distinct  crystalline  form  and 
definite  physical  properties.  A  very  large  number  of 
species  of  minerals  which  differ  greatly  from  each  other 
in  composition  and  physical  properties  have  been  recog- 
nized. It  is  these  differences  which  renders  necessary 
a  study  of  those  important  species  which  are  found  in 
the  soil,  in  order  to  gain  a  thorough  knowledge  of  the 
relations  which  they  bear  to  plant  nutrition  and  the  phys- 
ical and  chemical  characteristics  of  the  soil  mass.  By  their 
chemical  and  physical  weakness  or  resistance,  they 
modify  the  supply  of  food  elements  and  determine  the 
physical  make-up  of  the  soil,  with  all  the  attendant 
physical  conditions  of  heat,  moisture,  air,  etc.,  which 
this  limits. 

While  the  number  of  minerals  known  is  very  great, 
only  a  comparatively  small  number  occur  in  the  soil 
in  important  amounts;  but  these  are  thoroughly  repre- 
sentative. All  minerals  may  be  divided  into  two  groups: 
(1)  The  original  or  primary  constituents  which  were 
formed  at  the  first  consolidation.  (2)  The  secondary 
constituents  which  result  from  changes  in  the  minerals 
subsequent  to  their  first  consolidation,  and  which  are  due 
in  large  part  to  the  chemical  action  of  percolating  water. 

3.  Soil-forming  minerals;  their  composition  and 
properties. — The  soil  is  composed  of  a  great  variety 
of  minerals  and  probably  almost  every  recognized 
species  could  be  found  in  some  soil.  But  the  number 
of  minerals  which  make  up  the  bulk  of  soil  is  rela- 
tively small.  The  following  table  includes  the  most 
important  soil-forming  minerals  and  their  leading 
properties: 


i82 

Ol   CO 


g 

B 

£ 

2 

a 
1 


•t3   •%     O    £     O    TJ  • 

1     fllllpl     "I 

E     ^OS-2i«S| 


(NINMCOMCOCCM 


z 
y. 

1 

H 
X 

£ 

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as 


i 

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-/ 
I 

9 

o 

Q 


I 


o     o 


«r  S  w  E     «  tf  o 


-  S    - 


g 

• 

F^ 


(6) 


2 
I 

r 

f 


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5 

IQ 

•»   "            <-!•£  ^    <* 

o 

"r  —              * 

3  Q.                     fa 

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3  a       £  ^  * 

N 

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N 

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"32 

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Jd 

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Character  of  com- 
pound 

CaSO«-t-2HtO 

(CaMgFeMn),AltSuOn 

f 

I 

1 
i 

2 

FeOSHtO  +  FetO, 

J 

w 

O  r£ 

S  £ 

i 

M 

2MgO,  AUO,,  SiO,,  2HtO 
3Mgo,  4SiOiH,0 

Poly-silicates  +  HiO 
2HiO,  AUO»,2SiOi 

Poly-silicate  Fe  +  HjO 

i 

i 

•8 

Gypsum  .  . 

Garnet  .... 

Sodalite  .  .  . 

Hematite  . 

Magnetite.  . 

Limonite  .  .  . 

Halite  

•o  . 

4)  d  « 

d  a  .t! 

1*1 

V  4>  C 

r'C  as 
fl* 

Chlorite...  . 
Talc  

Zeolites.  .  .  . 
Kaolinite  .  . 

Glauconite 

55 

2 

«c 

-r 

t- 

DC 

~ 

^ 

—  N 
M  N 

N    W 

1C   CO 
»)    (M 

ft 

(7) 


8 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


4.  Relative  abundance  of  the  common  minerals. — 
Hall  quotes  D'Orbigny  as  saying  that  in  the  earth's 
crust  the  chief  minerals  are  present  in  the  following 
proportions: 

Feldspars 48 

Quartz 35 

Micas 8 

Talc 5 

Carbonate  of  lime  and  magnesium 1 

Hornblend,  augite,  etc 1 

All  other  minerals  and  weathered  products 2 

100 

These  general  relations  agree  with  the  statements 
of  Chamberlin  and  Salisbury,  who  give  the  following 
summary  of  the  salient  facts  relating  to  the  composition 

of  minerals:  "(1) 
Out  of  the  sev- 
enty-odd chemical 
elements  in  the 
earth,  eight  form 
the  chief  part  of 
it.  (2)  One  of 
these  elements 
uniting  with  the 
rest  forms  nine 
leading  oxides.  (3) 
One  of  these  ox- 
ides acts  as  an 
acid,  and  the  rest 
as  bases.  (4)  By 

Flo.  1.     Section  of  granite,  magnified.    The  _  .          . 

crystals    are    orthoclase,   microline,   plagioclase,  their  combination 

quartz,  black  mica  or  biotite,  white  mica  or  mus-  , 

covite.  (Merrill.)  they  form  a  series 


SOIL-FORMING  ROCKS  9 

of  silicates,  of  which  a  few  are  easily  chief.  (5)  These 
silicates  crystallize  into  a  multitude  of  minerals,  of 
which  again  a  few  are  chief.  (6)  These  minerals  are 
aggregated  in  various  ways  to  form  rocks." 

Hundreds  of  analyses  of  rocks  have  been  made  in 
this  country  and  abroad  and  from  these  Clark  finds  the 
mineralogical  composition  of  igneous  rocks  of  the  earth's 
crust  to  be  as  follows: 

Feldspars 59.5 

Hornblend  and  pyroxine    16.8 

Quartz 12.0 

Biotite  mica 3.8 

Titanium  minerals 1.5 

Apatite    0.6 

94.2 

This  leaves  5.8  per  cent  to  be  distributed  among  the 
more  rare  minerals. 

III.     IMPORTANT  SOIL-FORMING  ROCKS;  THEIR  PROPER- 
TIES   AND    OCCURRENCE 

A  rock  is  #n  aggregate  of  minerals.  Moreover,  it 
usually  exhibits  a  considerable  degree  of  consolidation, 
and  forms  an  essential  portion  of  the  earth's  structure. 
Very  few  minerals  occur  in  nature  in  largo  pure  masses. 
They  are  usually  grouped  together  in  different  combi- 
nations, and,  while  it  is  essential  to  trace  the  changes 
of  each  mineral,  it  is  also  necessary  to  give  attention  to 
the  groups  of  minerals — rocks — since  the  association 
of  minerals  determines  very  largely  the  processes  by 
which  rocks  are  transformed  into  soil  and  the  character- 
istics of  the  resulting  soil. 


10 


THE  PRINCIPLES  OF  SOIL   MANAGEMENT 


These  aggregates  of  minerals,  or  rocks,  are  essentially 
without  order  or  arrangement.  The  minerals  are  in 
irregular  crystals  or  fragments  of  greatly  differing  sizes 

closely  packed  to- 
gether. The  great 
variety  of  miner- 
als, as  well  as  the 
different  physical 
forms  of  the  same 
mineral,  is  pro- 
ductive of  an  infi- 
nite variety  of 
rocks.  While  in- 
dividuals may  dif- 
fer greatly,  there 
is  an  easy  and 
gradual  transition 
from  one  form  to 
another  which 
renders  it  impos- 
sible to  draw  hard 
and  well-defined  lines  separating  each  species  of  rock 
from  every  other  species.  They  blend  one  into  the 
other,  not  only  in  structure  and  crystalline  form  but 
also  in  chemical  composition. 

The  classification  of  rocks  is  based  upon  these  facts, 
and  they  are  grouped  broadly  under  four  main  heads, 
the  distinctions  being  their  origin  and  structure.  Each 
of  the  main  divisions  is  again  divided  into  groups  and 
families,  the  distinctions  being  those  of  mineral  and 
chemical  composition,  structure  and  mode  of  occurrence. 


FIG.  2.  Photomicrograph  of  diorite  rock.  Com- 
pare with  Figs.  3,  5,  and  6, 'which  have  a  different 
mineral  composition,  crystalline  form  and  struc- 
ture. These  differences  determine  the  type  and 
rate  of  their  weathering.  (Lord.) 


STRUCTURE  OF  IGNEOUS   ROCKS 


11 


The  main  divisions  are:  Igneous  rocks — sometimes 
called  eruptive — which  have  been  brought  up  from  below 
in  a  molten  condition  from  which  they  have  cooled  and 
solidified.  They  usually  have  two  or  more  essential 
minerals,  and  are  massive,  crystalline,  glassy,  or,  in 
certain  altered  forms,  colloidal  in  structure.  Aqueous 
rocks  have  been  formed  mainly  through  the  agency  of 
water,  as  (a)  chemical  precipitates,  or  as  (6)  sedimentary 
deposits.  They  are  usually  fragmental,  but  may  be 
crystalline  or  colloidal,  but  never  glassy.  They  have  a 
laminated  or  bedded  structure,  and  usually  have 
many  constituent  minerals.  ^Eolian  rocks  are  formed 
from  wind-drifted 
material.  They 
are  fragmental  in 
character  and  ir- 
regularly bedded 
in  structure.  Met- 
amorphic  rocks 
embrace  those  of 
any  of  the  fore- 
going divisions 
which  have  been 
changed  from 
their  original  con- 
dition through  the 
agencies  of  dyna- 
mic and  chemical 
forces  so  that  they  exhibit  new  properties.  They  may 
have  one  or  many  constituent  minerals,  and  in  structure 
they  are  usually  crystalline  and  bedded  or  foliated. 


Kio.  3. 


Photomicrograph  of  basalt   (trap)  rock. 
(Lord.) 


12 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


5.  Igneous,  aqueous,  aeolean  and  metamorphic 
rocks. — The  igneous  rocks  are  parent  to  all  the  other 
forms.  They  may  be  arranged  according  to  the  amount 
of  silica  they  contain,  those  that  are  rich  in  that  com- 
pound being  termed  acid,  and  those  that  are  lean, 
basic.  In  this  order,  some  of  the  most  abundant  rock 

types  are  granite, 
quartz,  syenites, 
diorites,  gabbro, 
diabase  and  ba- 
salts. 

Of  the  aqueous 
rocks  the  chemical 
precipitates  are 
relatively  of  small 
importance.  They 
seldom  form  ex- 
tensive rock 
masses  and  are 
usually  intimately 

FIG.  4.     Photomicrograph  of  fossiliferous  lime-        mingled        with 
-stone.    (Lord.)  other       types        of 

rock,  especially  those  of  the  sedimentary  group.  The 
most  important  ones  agriculturally  are  the  sulfates, 
represented  by  gypsum  beds.  Certain  phosphatic  deposits 
and  some  chlorides  also  belong  in  this  group. 

The  aqueous  sedimentary  rocks  are  the  most  import- 
ant agriculturally  of  any  of  the  groups  of  rock  and 
especially  of  the  aqueous  rocks,  because  of  their  'arge 
surface  distribution  and  their  physiography.  They  are 
composed  of  the  fragments  derived  from  the  degenera- 


STRUCTURE   OF  SEDIMENTARY   ROCKS 


13 


tion  of  all  the 
older  rocks  and 
from  the  inorganic 
remains  of  plant 
and  animal  life. 
These  comprise 
clay  and  shale  (ar- 
gillaceous), sand- 
stone, conglom- 
erate and  breccia 
(arenaceous)  ; 
limestone  and  dol- 
omite  (calcare- 
ous), together  with 
minor  rocks  of  vol- 
canic, phosphatic 
and  carbonaceous 
character. 

The  sand- 
stones, shales, 
limestone  and  dol- 
omite are  easily 
the  most  promi- 
nent of  this  group, 
and,  in  fact,  of  all 
the  types  of  rock, 
in  their  present 
agricultural  im- 
portance. They 
compose  immense 
strata  of  rock,  and 


FlQ.  5.     Photomicrograph  of  chert.    (Lord.) 


Fio.  6.     Photomicrograph  of  sandstone.    (Lord.) 


14  THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

are  usually  arranged  in  alternating  layers  of  variable 
thickness  and  extent,  and  have  given  rise  to  important 
areas  of  soil. 

JSolian  rocks  are  relatively  insignificant,  and  are 
generally  of  a  sandy  or  clayey  character. 

Metamorphic  rocks  are  correlated  with  all  the  other 
types  of  rock,  and  have  resulted  from  pronounced 
alterations  in  other  rocks.  Their  individual  properties 
are  therefore  similar  to  the  rock  from  which  they  were 
formed.  Often  their  resistance  to  decay  is  increased 
by  the  process,  as  in  quartzite  and  slate. 

IV.     CHEMICAL    AND    PHYSICAL    AGENCIES    OP 
ROCK-DECAY 

There  are  five  chief  agencies  of  rock-decay.  They 
are,  (a)  the  atmosphere,  (6)  heat  and  cold,  (c)  water, 
(d)  ice,  and  (e)  plants  and  animals.  The  operations  of 
each  of  these  agencies  are  of  two  sorts:  (1)  chemical; 
(2)  mechanical.  The  products  of  these  two  types  of 
force  are  distinctly  different  in  their  relaiton  to  the 
plant.  The  chemical  action  of  the  various  agencies 
results  in  a  changed  composition  of  the  minerals. 
It  results  in  the  breaking  down  of  the  mineral  com- 
pounds, with  the  possible  removal  of  the  elements,  as 
when  feldspar  is  changed  to  kaolinite.  Here  the  base — 
potash,  soda  or  lime — is  replaced  by  the  elements  of 
water,  and  may  be  carried  entirely  away.  The  hydrated 
residue  loses  some  of  its  silica,  and  kaolinite  is  the  result. 
In  other  cases  the  change  may  be  effected  by  the  addi- 
tion of  material,  as  when  pyrite  is  oxidized  by  the 


16  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

atmosphere  to  the  sulfate  by  the  direct  union  of  oxygen 
with  the  compound.  Whether  the  process  be  an  addition 
or  subtraction  of  material,  it  usually  changes  the  stabil- 
ity of  the  mineral,  and  perhaps  the  stability  of  the  mass 
of  which  the  mineral  is  a  part.  The  chemical  action  of 
one  agency  often  opens  the  way  for  the  chemical  and 
mechanical  action  of  other  agencies,  so  that  the  decay 
processes  are  hastened.  This  chemical  breaking  down 
of  minerals,  and  thereby  of  rock  masses,  is  termed 
decomposition.  The  mechanical  breaking  up  of  rocks 
whereby  only  the  state  of  division  of  the  material  is 
changed  is  termed  disintegration.  The  breaking  up  of 
rocks  due  to  expanison  of  heat,  the  freezing  of  water, 
flowing  of  water,  the  grinding  of  glacial  ice,  and  the 
expansion  of  plant  roots,  are  types  of  disintegration  by 
which  the  rock  is  simply  reduced  to  a  finer  state  of 
division.  The  general  tendency  is  for  finer  material  to 
result  from  decomposition  than  from  simple  disin- 
tegration. 

6.  Atmosphere. — The  atmosphere  is  composed  of  a 
mixture  of  the  gases  nitrogen  and  oxygen,  in  the  propor- 
tion of  four  parts  of  the  former  to  one  part  of  the  latter, 
together  with  very  minute  quantities  of  carbon  dioxide, 
nitric  oxide,  ammonia,  and,  in  even  less  amounts,  other 
volatile  compounds,  and  a  variable,  but  usually  very 
considerable  amount  of  water-vapor,  evidenced  by 
clouds,  rain,  snow,  dew,  etc.  These  gases,  dissolved  in 
the  atmospheric  moisture,  come  in  contact  with  rock 
masses  and  change  certain  of  its  minerals  into  com- 
pounds more  or  less  soluble  than  they  were  originally. 
The  iron  compounds  are  perhaps  the  most  affected, 


SOIL-FORMATION,    ATMOSPHERE  17 

md  the  change  of  the  mineral  pyrite  is  typical  of  the 

process. 

2Fe  Sa  +  7O2  +  2H2O  =  2Fe  SO4  +  2H2SO4 
Fe  S 


All  of  these  changes  of  iron  compounds  under  the 
action  of  moist  atmosphere  are  imperfectly  understood, 
but  it  is  agreed  that  the  above  products  may  result  from 
the  process.  Since  the  sulfate  is  much  more  soluble  than 
the  sulfid,  the  mineral  is  in  this  way  easily  removed. 

The  purely  chemical  action  of  the  atmosphere  is  less 
pronounced  in  its  effects  than  its  mechanical  action. 
As  wind,  it  exerts  some  pressure  upon  projecting  masses 
tending  to  push  them  over,  but  its  great  work  is  accom- 
blished  when  the  wind  carries  solid  particles  of  dust  and 
sand  and  when  it  acts  on  vegetation  as  a  lever.  In  arid 
and  semi-arid  regions,  particularly,  the  amount  of  solid 
material  carried  in  the  atmosphere  is  very  large  at 
some  seasons.  There  frequently  occur  dust  storms, 
when  the  atmosphere  is  so  filled  with  wind-driven  par- 
ticles as  to  obscure  the  sun  and  all  objects,  at  even  a 
short  distance  away.  In  the  region  of  western  Nebraska 
and  Kansas  these  dust  storms  are  well  known,  and 
on  certain  soils  it  is  unwise  to  plow  in  the  fall,  because 
by  spring  the  soil  will  have  been  blown  away  to  the 
depth  of  the  furrow,  and  indeed  this  sometimes  results 
from  plowing  at  any  other  season  of  the  year.  Further 
west  in  the  mountain  region  this  wind-blown  material 
is  most  effective,  where  the  particles  may  be  driven 
against  the  bare  rock  faces.  It  then  becomes  a  titanic 
sand  blast  to  drill  away  the  rock.  It  eats  into  the  rock 
surface  with  remarkable  rapidity,  carving  fantastic 


18  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

forms,  as  a  result  of  the  varying  hardness  of  the  rock 
and  the  uneven  distribution  of  the  particles.  The 
abraded  particles  are  born  along  by  the  wind  and  be- 
come new  tools  of  destruction. 

In  humid  regions  this  form  of  disintegration  is  less 
prominent,  but  in  sandy  regions  it  performs  some  ef- 
fective work.  As  an  example  of  this  effectiveness, 
Merrill  describes  a  large  sheet  of  plate-glass,  once  a 
window,  in  a  lighthouse  on  Cape  Cod, — well  known  for 
its  sand-dunes.  During  a  severe  storm,  of  not  above 
forty-eight  hours'  duration,  this  became  on  its  exposed 
surface  so  ground  by  the  impact  of  grains  of  sand 
blown  against  it  as  to  be  no  longer  transparent,  and  to 
necessitate  its  removal.  He  reports  that  window-panes 
in  dwelling-houses  in  the  vicinity  are  frequently  drilled 
quite  through  by  the  same  means. 

Material  blown  about  by  wind  is  very  much  rounded 
and  smoothed  by  the  impacts  to  which  it  has  heen  sub- 
ject, a  characteristic  very  much  less  in  evidence  in 
water-moved  material  of  the  same  fineness. 

Winds  also  act  in  conjunction  with  plants  where  the 
roots  have  penetrated  into  a  crevice  or  joint,  using  the 
tops  as  a  lever  to  push  off  or  further  fracture  masses  of 
rock.  This  process  is  most  effective  in  rough  mountainous 
regions  where  the  larger  vegetation  is  just  getting  a  foot- 
hold. In  passing,  attention  may  be  called  to  this  process 
of  overturning  plants  as  one  of  nature's  cultural  methods, 
whereby  the  soil  is  subjected  to  very  thorough,  if  long- 
drawn-out,  tillage. 

7.  Heat  and  cold. — In  general,  heat  accelerates  all 
chemical  processes.  It  greatly  increases  the  solvent 


SOIL-FORMATION,    HEAT   AND   COLD 


19 


power  of  water  for  many  substances,  and  renders  it  a 
more  destructive  agent  generally.  This  action  can  not 
be  discussed  separately,  but  must  be  kept  in  mind  in 
the  consideration  of  those  other  agencies  of  decompo- 


Fio.  8.     Two  types  of  rock  disintegration.    The  forms  reflect  the  different 
hardness  and  composition  of  the  rocks 

sition.  Especially  important  are  alterations  of  tempera- 
ture, by  which  compounds  whose  rates  of  solution  are 
differently  affected  by  temperature  may  be  successively 
acted  upon. 

Heat   acts   mechanically  in   two  ways  to  break   up 
rocks    (1)  Through  expanison  and  contraction  due  to 


20  THE  PRINCIPLES   OF   SOIL   MANAGEMENT 

changes  in  temperature.  All  substances  change  volume 
with  changes  in  temperature.  Different  minerals  expand 
at  different  rates,  and  the  same  mineral  may  have 
different  rates  of  expansion  along  different  axes.  So 
that,  when  a  rock  made  up  of  several  minerals  has  its 
temperature  changed,  it  expands  unequally,  and  a 
strain  is  set  up  all  through  the  mass,  which,  if  severe 
enough,  and  repeated  often  enough,  will  break  it  into 
small  fragments.  Further,  even  if  a  rock  did  expand 
uniformly  in  all  its  parts  with  changes  of  temperature, 
these  changes  of  temperature  are  far  from  uniform. 
Heat  is  conducted  slowly  into  a  rock.  Since  the  rock 
may  have  very  different  temperatures  at  points  a  short 
distance  apart,  as  a  result  of  this  slight  conductivity 
a  great  strain  may  result  from  expansion  due  to  tem- 
perature differences.  Merrill  quotes  Bartlett  to  the  effect 
that  granite  expands  .000004852  inch  per  foot  for  each 
degree  of  Fahr.,  marble  .000005668  inch,  and  sandstone 
.000009532  inch.  While  these  movements  appear 
exceedingly  small,  they  are  multiplied  through  many 
feet  of  rock  and  through  many  degrees  of  temperature. 
The  differences  in  temperature  between  day  and  night 
on  rock  surfaces  exposed  to  the  sun  is  extreme,  although 
it  varies  with  the  color  of  the  rock.  (2)  When  water  is 
carried  below  its  freezing-point,  it  may  be  exceedingly 
destructive.  In  freezing,  water  expands  about  one- 
eleventh  of  its  volume.  It  has  been  determined  that 
water  at  a  temperature  of — 1°C.  exerts  an  expansive 
force  of  150  tons  per  square  foot,  and  that  to  keep  it 
from  becoming  ice  would  require  the  weight  of  a  column 
of  granite  1,800  feet  high.  All  rocks  are  somewhat 


SOIL-FORMATION,    WATER  21 

porous.  Soils  have  a  porosity  anywhere  from  30  to  75 
per  cent  of  their  volume.  Sandstone  may  have  as  much 
as  25,  limestone  from  .1  to  .01.  marble  .008,  and  granite 
.01  per  cent.  If  this  spore  space  is  filled  with  water,  as 
is  generally  the  case  in  nature,  and  the  rock  is  cooled 
below  the  freezing-point,  it  is  evident  that  it  will  be 
shattered.  As  the  process  is  repeated,  the  fractures 
become  larger  and  more  numerous. 

8.  Water.  —  The  chemical  and  mechanical  action  of 
water  in  rock-decay  may  be  discussed  separately. 

(1)  The  chemical  action  may  be  divided  into:  (a) 
The  changes  due  to  pure  water,  (b}  Changes  due  to 
material  in  solution  in  the  water.  Owing  to  the  porosity 
of  rocks,  water  is  distributed  through  all  the  earth's 
crust  to  a  depth  of  many  thousand  feet. 

The  first  direct  result  of  the  presence  of  water  is  the 
assumption  of  its  elements  by  many  of  the  minerals. 
This  is  hydration.  It  may  be  the  direct  imbition  of 
water,  as  when  calcium  sulfate  in  crystallizing  takes 
into  its  constitution  several  molecules  of  water;  or  it 
may  be  the  substitution  of  the  elements  of  water  for 
some  elements  already  in  the  mineral.  The  alterations 
in  the  mineral  orthoclase  feldspar  may  be  taken  as  the 
type  of  this  kind  of  changes  as  follows: 


K,0,  A120,,  6SiO,  +  6  HJO  =  2KOH  +  H.1O,  A12O3,  6SiO3,  4H2O 

Since  water  is  so  widely  diffused,  this  process  of 
hydration  is  an  especially  important  one.  The  signifi- 
cant chemical  effect  of  hydration  is  that  it  alters  the 
solubility  of  the  mineral,  and  particularly  of  the  elements 
composing  the  mineral. 


22 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


The  second  direct  chemical  action  of  water,  and 
perhaps  the  most  important  of  all  the  chemical  changes 
involved  in  soil  formation,  is  that  of  solution.  It  is 
worth  while  to  remember  that  no  mineral  is  completely 


FIG.  9.     Traces  of  residual  soil  in  a  limestone  quarry.    Note  the  joints  and 
partings.   Soil  of  a  dark  red,  silty  character 

insoluble.  They  differ  greatly  in  solubility,  ranging  from 
the  readily  soluble  common  salt  to  the  exceedingly 
insoluble  silica  or  quartz.  But  all  are  amenable  to  the 
action  of  pure  water.  In  the  above  instance  of  hydration 
of  feldspar,  we  have  a  type  of  a  large  number  of  changes 
in  minerals  which  alter  their  solubility.  And  by  altering 
the  solubility  of  one  mineral  the  other  minerals  present 


SOIL-FORMATION,    WATER  23 

are  opened  to  attack  by  any  one  of  many  agencies,  both 
mechanical  and  chemical.  In  feldspar,  which  is  very 
slightly  soluble,  hydration  and  hydrolysis  develops 
potassium  hydrate,  a  very  soluble  compound  and  there- 
fore readily  removed.  Its  removal  may  develop  a  cavity, 
and  thus  weaken  the  rock.  Agriculturally,  the  removal 
of  the  base  is  also  significant.  It  is  the  basic  element, 
and  therefore  largely  plant-food  elements,  or  those 
which  condition  soil-productiveness,  such  as  potash, 
lime  and  soda,  which  are  removed  by  this  process. 

It  is  because  of  the  unequal  solubility  of  minerals 
that  soil  results  from  the  process  of  solution.  If  all  the 
minerals  of  a  rock  were  equally  soluble,  the  rock  might 
be  removed  bodily  from  the  exposed  surface  inward. 
Solubility,  operating  differently  for  different  minerals 
in  a  rock  mass,  removes  one,  and  leaves  the  others  in  a 
less  coherent  mass,  which  we  term  soil.  It  therefore 
happens  that  residual  soils  comprise  the  less  soluble 
portions  of  the  rock  from  which  they  were  formed. 

Materials  in  solution  in  water  greatly  affect  its 
capacity  to  dissolve  minerals.  Carbon  dioxid  is  present 
in  the  air  in  the  pores  of  rocks  and  soils  in  much  larger 
proportion  than  in  the  air  above  the  earth's  surface. 
It  is  particularly  abundant  in  the  surface  layers,  where 
it  is  derived  from  organic  decay.  It  is  taken  up  by  the 
water  as  it  passes  along  and  becomes  a  means  of  solution. 
The  most  striking  example  of  this  is  in  the  case  of  lime 
carbonate  or  limestone.  In  pure  water  this  mineral  is 
soluble  only  to  the  extent  of  about  one  part  in  twenty-five 
thousand,  but  in  carbonated  water  its  solubility  is 
about  one  part  in  one  thousand,  or  twenty-five  times  as 


24  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

soluble.  It  is  this  solvent  action  of  carbonated  water 
which  has  formed  the  extensive  caverns  and  passages 
in  every  fairly  pure  limestone  formation,  and  thereby 
has  given  rise  to  such  features  as  the  Mammoth  Cave 
and  the  great  sinks  of  Southern  Missouri,  Kentucky, 
Tennessee,  Georgia,  Florida  and  many  other  regions 
underlain  by  limestone. 

In  the  superficial  layers  of  soil,  organic  acids  also 
add  to  the  solvent  power  of  the  water. 

Since  water  is  an  universal  solvent,  in  the  earth  it 
contains  a  large  variety  of  mineral  compounds,  all  of 
which  affect  its  solvent  power,  usually  increasing  it. 

The  destructive  action  of  solution  is  indicated  by  the 
considerable  amount  of  dissolved  substances  in  all 
natural  waters.  The  Mississippi  river  carries  in  solution 
annually  sufficient  material  to  cover  a  square  mile  of 
land  ninety  feet  deep;  the  Danube,  sufficient  material 
for  a  depth  of  eighteen  feet;  and  the  Nile,  sufficient 
material  for  a  depth  of  thirteen  feet. 

(2)  The  mechanical  action  of  water. — The  destructive 
action  of  running  water  transcends  all  other  agencies 
of  rock  degradation  in  its  extent.  It  has  been  the  most 
potent  force  in  carving  the  earth's  surface  into  its  present 
form.  It  is  continually  at  work  reducing  elevations 
and  filling  depressions. 

This  destructive  action  is  due  largely  to  the  power 
of  running  water  to  carry  material.  This  transported 
material  becomes  the  tool  of  the  water  in  wearing  away 
its  channel. 

The  transporting  power  of  water  varies  as  the  sixth 
power  of  its  velocity  of  flow.  That  is  to  say,  if  the 


SOIL-FORMATION,    WATER 


25 


Fio.  10.  "Pot  holes"  formed  in  shale  rock.  The  boulders  ana  pebbles 
in  the  "pot"  are  set  in  motion  by  flowing  water  and  thereby  the  rock  is  broken 
down  with  the  formation  of  soil  material. 

velocity  of  a  stream  is  doubled,  its  carrying  power  will 
be  increased  sixty-four  times.  But  the  volume,  and 
therefore  the  weight,  of  a  body  varies  as  the  cube  of  its 
diameter.  Therefore  the  diameter  of  the  material 
carried  does  not  vary  directly  as  the  velocity  of  the 
current  but  at  a  less  rate. 

This  power  of  flowing  water  to  carry  rock  material 


26  THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

is  exemplified  in  every  stream  of  whatever  size.  Where 
the  flow  is  checked  and  thereby  the  carrying  power 
reduced,  some  of  the  coarsest  material  is  deposited. 
Where  the  flow  is  increased,  instead  of  deposition, 
coarser  material  is  picked  up.  Changing  an  obstruction 
causes  extensive  regrading  of  the  channel  by  the  current. 
Bends  in  the  stream  which  require  a  greater  velocity  on 
one  side  of  the  channel  than  on  the  other  cause  the  same 
sort  of  rearrangement,  and  this  is  nicely  illustrated 
in  the  meandering  of  streams.  They  wind  over  their 
course  always  cutting  away  the  material  on  the  outer 
side  of  the  curves,  and  depositing  it  on  the  inner  side 
of  the  curves  lower  down.  Thus  the  stream  is  continually 
changing  its  course.  It  meanders  from  one  side  of  its 
flood-plane  to  the  other.  It  cuts  off  large  curves  and 
proceeds  to  form  new  ones.  All  these  processes  may 
be  observed  in  any  rivulet,  yet  they  are  the  exact 
counterpart  of  the  things  which  are  taking  place  in 
every  large  river  valley.  Careful  determinations  re- 
ported by  Bobb,  show  that  the  large  rivers  of  the  world 
remove  annually  in  suspension  the  following  amounts 
of  material: 

Height  in  feet  of  Thickness  of  sedi- 

column  of  sedi-  ment,  in    inches 

ment  with  base  if     spread    over 

1  mile  square.  drainage  area. 

Mississippi  river   241.4  .00223 

Potomac 4.0  .00433 

Rio  Grande 2.8  .00116 

Uruguay 10.6  .00085 

Rhone 31.1  .1075 

Po 59.0  .01139 

Danube 93.2  .00354 

Nile 38.8  .00042 

Mean  . .  .   76.65  .00614 


SOIL-FORMATION,   ICE  27 

In  addition  to  the  material  carried  in  suspension, 
a  large  amount  is  rolled  along  the  bottom  of  the  channel. 

Because  of  the  unequal  carrying,  power  of  streams 
of  different  velocity,  the  load  of  debris  is  sorted  into 
groups  of  somewhat  uniform  size.  In  this  way  have 
been  formed  great  areas  of  clay,  silt,  sand  and  gravel 
found  in  all  farming  sections,  and  which  owe  their 
peculiar  crop-producing  properties  most  largely  to  this 
sorting  action  of  water. 

9.  Ice — glaciers. — Masses  of  ice  haye  exerted  a 
tremendous  influence  in  the  reduction  of  rocks  to  soil 
material.  Their  action  is  chiefly  mechanical,  but  is  inti- 
mately associated,  as  a  rule,  with  the  action  of  water. 
The  chief  agency  of  ice  is  in  the  form  of  glaciers,  which 
issue  from  regions  of  high  latitude,  or  of  great  elevation, 
and  in  times  past  have  pushed  down  over  much  larger 
areas  of  country  than  they  now  occupy.  A  large  part 
of  all  of  the  continents  have  been  overrun  by  such 
masses,  which,  through  their  great  weight  and  almost 
resistless  movement,  ground  even  the  hardest  rocks  to 
fine  powder  and  mixed  the  materials  from  many  sources. 
Fragments  of  rock  imbedded  in  the  bottom  of  the  ice 
became  its  tools  to  scratch  and  crush  the  floor  upon 
which  it  rested.  In  this  way  has  come  about  the  scouring 
and  pulverization  of  rocks,  analogous  to  the  action  of 
water.  The  ice  appears  to  have  attained  a  depth  of 
thousands  of  feet  in  some  places,  and  consequently  was 
able  to  override  even  mountainous  areas,  sweeping  away 
and  grinding  to  fragments  the  smaller  eminences  and 
irregularities.  Since  the  access  of  water  was  limited, 
there  was  little  opportunity  for  pronounced  chemical 


28 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


change,  or  the  removal  of  constituents,  which  fact  is 
shown  in  the  tables  of  soil-composition  on  pages  32-57. 
Their  influence  on  .surface  topography  is  profound,  and 
of  great  importance  to  the  pursuit  of  agriculture  because 
of  the  leveling  which  results. 


FIG.  11.  Lichen  growing  on  a  granite  boulder.  These  low  forms  of 
plants  disintegrate  the  rock  and  assist  in  the  decomposition  of  its  constituent 
minerals 

10.  Plants  and  animals. — Plants  and  animals  unite 
with  the  other  agencies  mentioned  to  effect  the  breaking 
down  of  minerals  and  rocks  Like  the  other  processes, 
they  have  both  their  mechanical  and  their  chemical 
side.  The  development  of  plant  roots  in  crevices  of  rock 


SOIL-FORMATION,    PLANTS   AND   ANIMALS 


29 


created  by  other  agencies,  exerts  sufficient  pressure  to  force 
them  further  apart  and  extend  the  fractures.  Occasional 
striking  examples  of  the  forcing  apart  of  rock-masses 
by  plant-growth  may  be  observed.  The  process  is  well 


Fio.  12.    Growing  roots  of  the  tree  have  broken  apart  and  otherwise  disinte- 
grated the  Kranite  boulder,  thereby  assisting  in  the  formation  of  "oil 

illustrated  by  the  lifting  of  sidewalks  and  the  tipping 
over  of  stone  fences,  due  to  the  development  of  trees 
near  by  and  even  such  soft  tissues  as  those  of  mush- 
rooms have  been  observed  pushing  up  through  cement 


30  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

and  brick  sidewalks.  In  mountainous  regions  wnere 
vegetation  has  gained  a  foothold  in  the  crevices,  the 
tops  serve  the  wind  as  a  lever  to  pry  rocks  apart.  The 
overturning  of  trees  is  a  familiar  example  of  the  process. 
Animal  life  also  has  a  part  in  the  mechanical  breaking 
down  of  rocks.  Burrowing  animals  are  most  active. 
The  gopher,  the  prairie  dog,  the  badger,  the  rabbit, 
moles,  etc.,  all  burrow  in  the  ground  and,  in  the  aggre- 
gate, move  large  masses  of  material.  Cray-fish  and 
earth-worms  are  even  more  widespread,  and  the  latter 
by  their  large  numbers  have  a  capacity  which  is  likely 
to  be  underrated  because  it  is  largely  out  of  sight. 
Ants  are  another  very  active  form  of  animal  life  in 
effecting  soil  formation.  They  burrow  into  crevices  of 
rocks  and  into  soil  formations,  and  deposit  the  material 
from  the  passages  at  the  surface  mixed  with  their  acid 
saliva.  Like  the  earth-worms,  they  handle  immense 
amounts  of  material. 


V.     GEOLOGICAL    CLASSIFICATION    AND    CHEMICAL 
COMPOSITION    OF    SOILS 

All  soil  material  may  be  divided  into  two  groups, 
depending  upon  the  extent  to  which  it  has  been  moved 
in  the  process  of  formation.  Those  materials  which  have 
not  been  subject  to  any  appreciable  transportation 
are  termed  (a)  Sedentary.  Those  which  have  been 
carried  to  their  present  position — that  is,  have  been 
appreciably  moved — are  termed  (6)  Transported.  There 
are  several  agencies  of  transportation,  such  as  gravity, 
water,  ice  and  wind.  These  give  rise  to  subdivisions. 


SEDENTARY   SOILS  31 

11.  Sedentary    soils. — Sedentary    soils    are    of     two 
kinds:   (1)  Residual,  or  soils  consisting  of  the  residue 
left  behind  in  rock  decomposition.     (2)  Cumulose,   or 
soils  resulting  from  the  slow  accumulation  and  decay  of 
plant  remains. 

12.  Residual   soils. — There  may  be  as  many   kinds 
of  residual  soil  as  there  are  rocks.    Because  of  similarity 
between  the  species  in  a  group  of  rocks,  a  few  of  these 
groups  may  be  considered  as  types.    The  most  promi- 
nent groups  are  the  igneous  rocks,  the  calcareous  rocks, 
shale  or  slate  and  sandstone.    Attention  will  be  directed 
as  far  as  possible  to  the  relation  of  the  soil  composition 
to  the  composition  of  the  original  rock  and  to  the  char- 
acter of  the  material  lost  in  the  transition. 

In  calculating  the  relative  loss  of  the  different  ele- 
ments in  the  transition  process,  some  one  element- 
usually  iron  or  aluminum,  and,  in  the  case  of  limestone, 
silicon — is  assumed  to  have  suffered  no  loss.  This 
method,  adopted  from  Merrill,  is,  of  course,  not  strictly 
accurate,  since  every  element  is  subject  to  losses;  but 
it  serves  as  a  fair  comparative  basis  for  the  study  of  the 
loss  of  plant-food  elements. 

The  important  areas  of  residual  soil  in  North  America 
occur  south  of  the  limit  of  glaciation,  which  extends 
roughly  from  New  York  to  Cincinnati,  thence  to  St. 
Louis  and  up  the  Missouri  river  to  the  Dakotas,  and 
west  to  the  Sound  region  of  Washington,  where  it  again 
loops  well  to  the  south.  The  residual  soils  are  further 
hemmed  in  by  coastal  deposits,  which  have  their  greatest 
extent  in  the  South  Atlantic  and  Gulf  Coast  region, 
where  they  reach  a  width  of  more  than  a  hundred  miles. 


32 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


TABLE   II.  —  COMPLETE    CHEMICAL    COMPOSITION    OF    ROCKS 
AND    RESIDUAL    SOILS. 


Granite. 
District  of 
Columbia 

Gneiss. 
Albemarle 
Co.,  Va. 

Diabase. 
Spanish 
Guiana 

Basalt. 
Crouzet 
Haute  Loire, 
France 

I 

II 

Ill 

IV 

V 

VI 

VII 

VIII 

Re- 

Re- 

Re- 

Re- 

Fresh 

sidual 

Fresh 

sidual 

Fresh 

sidual 

Fresh 

sidual 

Rock 

Sand 

Rock 

Clay 

Rock 

Clay 

Rock 

Soil 

1.  Silica  (SiO2)  .. 

69.33 

65.69 

60.69 

45.31 

49.35 

43.38 

48.29 

37.09 

2.  Alumina 

(A1203)  .... 

14.33 

15.23 

16.89 

26.55 

15.30 

18.36 

13.25 

30.75 

3.  Ferric   iron 

(Fe203)  .... 

4.00 

4.39 

9.06 

12.18 

14.25 

20.39 

17.12 

4.31 

4.  Ferrous  iron 

(FeO)  

5.  Sulfur  trioxid 

(SO,) 

6.  Phosphoric 

acid  (P2O-). 

0.10 

0.06 

0.25 

0.47 

7.  Lime  (CaO)  .  . 

3.21 

2.63 

4.44 

t 

9.60      2.37 

7.37 

8.97 

8.  Carbon  Dioxid 

(CO2)  

9.  Magnesia 

(MgO)  

2.44 

2.64 

1.06 

0.40 

7.38 

3.45 

7.03 

0.61 

10.  Soda  (Na2Q). 

2.70 

2.12 

2.82 

0.22 

1.98 

0.14 

2.71 

1.01 

11.  Potash  (K2O). 

2.67 

2.00 

4.25 

1.10 

0.85 

0.59 

1.81 

0.71 

12.  Ignition    [wa- 

ter (H20)]... 

1.22 

4.70 

0.62 

13.75 

3.25 

11.34 

4.92 

16.55 

99.60 

99.77 

In  addition  to  these  large  areas,  many  small  areas  occur 
scattered  through  areas  of  other  kinds  of  soil. 

The  most  nearly  original  soil  is  that  formed  from 
igneous  rocks.  That  is  to  say,  the  composition  of  such 
a  soil  might  be  expected  to  approach  most  nearly  to 
that  of  the  original  rock.  The  relative  composition  of 
several  igneous  rocks  and  the  soils  derived  from  them, 


CHEMICAL   COMPOSITION   OF   RESIDUAL   SOILS       33 


TABLE  II.  —  COMPLETE   CHEMICAL   COMPOSITION  OF  ROCK  AND 
RESIDUAL    SOILS,   continued. 


Soapstone 

Igneous 
Rock 

Sand- 
stones 

Shales 

Limestone 

Albe 
Count 

IX 

Fresh 
Rock 

•narle 

f,    Va. 

X 

Re- 
sidual 
Soil 

Aver- 
age of 
about 
700 
sam- 
ples 

Com- 
posite 
analy- 
ses  253 
sam- 
ples 

Com- 
posite 
analy- 
ses 78 
sam- 
ples 

Com- 
posite 
analy- 
ses  345 
sam- 
ples 

Carboniferous, 
Arkansas 

XI 

XII 

XIII 

XIV 

XV 

Fresh 
Rock 

XVI 

Re- 
sidual 
Clay 

1.  Silica  (SiO3)  .. 
2.  Alumina 

(A1303)    ... 
3.  Ferric  iron 

(FeA).... 
4.  Ferrous  iron 
(FeO)  

38.85 
12.77 
12.86 

38.82 
22.61 
13.33 

59.87 
15.02 
2.58 
3.40 
0.28 

0.26 
4.79 

0.52 

4.06 
3.39 
2.93 

1.86 

78.66 
4.78 
1.08 
0.30 
0.07 

0.08 
5.52 

5.04 

1.17 
0.45 
1.32 

1.64 

58.38 
15.47 
4.03 
2.46 
0.65 

0.17 
3.12 

2.64 

2.45 
1.31 
3.25 

5.02 
MnO 

5.19 
0.81 
0.54 

4.13 
4.19 
2.35 

33.69 
30.30 
1.99 

5.  Sulfur  trioxid 
(SO,).. 

0.05 

0.04 
42.61 

41.58 

7.90 
0.05 
0.33 

0.77 
0.05 

6.  Phosphoric 
acid  (P3O5)- 
7.  Lime  (CaO)  .  . 
8.  Carbon  dioxid 
(CO3)  

3.04 
44.79 

34.10 

0.30 
0.16 
0.35 

2.26 
4.33 

2.54 
3.91 

0.26 
0.61 
0.96 

10.76 
14.98 

6.12 

6.13 

9.  Magnesia 
(MgO)  
10.  Soda  (Na2O)  . 
11.  Potash  (K3O)- 
12.  Ignition  [wa- 
ter (H20)]  . 

22.58 
0.11 
0.19 

6.52 

9.52 
0.20 
0.18 

9.21 

as  given  by  Merrill,  is  shown  in  Table  II,  numbers  I  to 
VIII.  Numbers  I  and  II  represent  a  gray  foliated 
granite  from  the  District  of  Columbia,  the  soil  of  which 
is  very  sandy.  By  reference  to  Column  II  of  Table  III, 


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36  THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

it  will  be  seen  that  the  total  loss  from  the  rock  is  13.47 
per  cent.  Column  III  of  Table  III  represents  a  gneiss  from 
Albemarle  county,  Virginia,  under  almost  the  same 
climatic  conditions  as  the  granite.  But  the  soil  is  a  red 
clay  of  the  Cecil  series,  and  represents  a  loss  in  transition 
from  the  rock  of  44.07  per  cent,  or  three  and  one-half 
times  as  much  as  from  the  granite.  The  composition  of 
the  two  rocks  is  not  greatly  different.  The  differences 
in  the  two  soils  illustrate  the  two  types  of  rock-decay. 
The  granite  soil,  which  is  very  sandy,  probably  does 
not  represent  the  same  advanced  stage  of  decay  as  the 
gneiss  soil,  and  apparently  has  been  subjected  most 
largely  to  disintegration,  or  physical  breakdown.  On 
the  other  hand,  the  gneiss  soil  represents  both  the  disin- 
tegration and  an  advanced  stage  of  chemical  change  or 
decomposition. 

In  general,  the  productiveness  of  a  soil  depends 
even  more  on  its  physical  characteristics  than  on  its 
chemical  composition.  The  physical  characteristics  of 
a  residual  soil  depend  quite  as  much  on  the  stage 
and  type  of  decay  to  which  it  has  been  subject  as  to 
its  chemical  composition.  Mechanical  processes,  such 
as  abrasion  and  fracture  due  to  impact,  temperature 
changes  and  frost,  never  produce  the  same  fine  texture 
which  may  result  from  chemical  processes,  and  therefore 
such  material  is  usually  very  sandy.  A  sand  composed  of 
aluminum  silicate  minerals  in  large  proportion  is  increas- 
ingly subject  to  chemical  decay,  which  will  reduce  it  to 
a  gritty  clay  of  progressive  coarseness  from  the  surface 
downward.  These  principles  may  be  summed  up  in  the 
statement  that  the  characteristics  of  a  soil  are  determined 


COMPOSITION   OF   RESIDUAL   SOILS  37 

by  two  factors:  (1)  The  original  chemical  and  physical 
composition  of  the  rock.  (2)  The  relative  prominence  of 
physical  and  chemical  processes  in  its  formation.  These 
facts  make  possible  the  existence  of  a  full  series  of  soil 
from  any  group  of  rocks. 

The  composition  of  other  rocks  and  soils  than  those 
mentioned  above  are  shown  in  Columns  V  to  X  of  Table 
II.  For  comparative  purposes,  Column  XI  is  also  of 
great  interest,  as  showing  the  average  composition  of 
over  700  bulk  analyses  of  igneous  rocks  as  given  by  Clark. 
This  gives  some  idea  of  the  relative  abundance  of  the 
several  plant-food  constituents  in  the  rocks.  It  will  be 
noted  that  the  least  abundant  elements,  sulfur  and 
phosphorus,  are  present  in  amounts  of  several  thousand 
pounds  per  acre  foot. 

Columns  XII,  XIII  and  XIV  give  the  analysis  of  a 
composite  of  many  samples  of  sandstone,  shales  and 
limestones.  The  first  two  may  be  considered  as  ancient 
soils,  and  their  average  composition  of  the  mineral 
elements  should  be  much  the  same  as  modern  soils  of 
the  same  origin. 

Columns  XIV  to  XVI  give  the  composition  of  lime- 
stones, and  of  a  residual  soil  from  such  a  rock  in  Ar- 
kansas. From  a  comparison  of  the  first  two  columns, 
it  will  be  found  that  the  rock  from  which  the  soil  is 
derived  is  far  from  the  average,  especially  in  the  amounts 
of  manganese  and  phosphorus  it  carries.  A  study  of 
the  soil  analysis  also  shows  that,  while  it  is  derived 
from  a  lime  rock,  it  is  not  rich  in  lime,  a  condition  not 
uncommon. 

Turning  now  to  Table  III,  there  is  given  the  propor- 


38 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


tion  of  loss  of  the  different  elements  calculated  to  the 
amount  of  the  element  originally  present,  and  to  the 
proportion  the  loss  bears  to  the  original  rock.  This 
exhibits  some  of  the  reasons  for  the  difference  between 


Fia.  13.     Residual  soil  from  lir 


Showing  relation  to  underlying  rock 


many  soils  and  the  rocks  from  which  they  were  derived. 
Assuming  that  there  is  any  element  which  is  constant 
in  amount,  these  figures  show  that  the  total  loss  suffered 
by  different  rocks  ranges  from  97.64  per  cent  for  the 
limestone  to  13.47  for  the  granite.  In  other  words,  a 
limestone  soil  represents  the  supplementary  materials 
in  the  original  rock,  the  main  constituent  having  been 


LOSSES   IN   RESIDUAL   SOIL   FORMATION  39 

removed.  In  this  particular  sample,  100  feet  of  rock 
would  produce  only  2.34  feet  of  soil.  It  is  not  uncommon 
in  limestone  soil  regions,  as  Kentucky,  Tennessee  and 
the  Ozark  region,  to  find  soils  forty  and  more  feet  in 
depth,  and,  since  the  average  limestone  contains  nearly 
90  per  cent  of  carbonate,  these  deep  layers  of  soil  must 
represent  some  hundreds  of  feet  of  rock.  This  is  the  re- 
sult almost  entirely  of  solution  by  carbonated  waters, 
which  gradually  develop  crevices  and  caverns  in  the 
rock. 

Other  types  of  rock,  however,  do  not  suffer  such  a 
large  amount  of  loss.  The  loss,  of  course,  varies  with 
the  character  of  the  processes  which  are  at  work,  as 
has  been  pointed  out  in  the  case  of  granite  and  gneiss. 
In  Columns  V  and  VI,  a  clay  from  diabase  rock  suffered 
a  loss  of  39.51  per  cent,  and  a  basalt  soil  in  France  rep- 
resented a  loss  of  over  60  per  cent.  The  latter  are  much 
more  -basic  than  the  granite  or  gneiss,  and  would  there- 
fore be  more  amenable  to  chemical  decay.  The  soap- 
stone,  which  results  from  the  alteration  of  pyroxinite 
rock,  undergoes  a  loss  of  52  per  cent  in  the  transition 
to  soil.  In  Columns  XV  to  XVIII  are  given  the  calcu- 
lated loss  in  changing  from  the  average  analysis  of 
igneous  rocks  to  shale  and  sandstone  respectively.  As 
was  stated  above,  these  latter  are  ancient  soil  material, 
or  potential  soil  material,  and  the  figures  given  represent 
an  attempt  to  determine  the  average  change  which  takes 
place  in  the  derivation  of  a  shale  or  sandstone  (corre- 
sponding to  clay  or  sand  soil)  from  igneous  rocks.  These 
calculations  are,  of  course,  less  accurate  than  the  pre- 
vious figures  on  such  loss,  because  these  rocks  have  been 


40  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

subject  to  mechanical  sorting 'by  wind  and  water,  in 
addition  to  the  fact  that  no  single  element  has  come 
through  without  loss. 

The  figures  in  the  first  column  of  each  pair  show  the 
proportionate  loss  of  each  constituent.  The  second 
column  shows  what  would  be  expected,  viz.,  that  the 
elements  present  in  largest  amount  would  be  subject 
to  the  largest  total  loss.  But  the  first  column  shows 
that  certain  elements  are  more  weak  chemically  than 
others.  These  elements  are  lime,  magnesium  and  the 
alkalies.  While  the  figures  are  limited,  still  phosphoric 
acid  appears  to  be  subject  to  a  large  loss.  There  is  almost 
invariably  the  assumption  of  water,  and  frequently  of 
carbon  dioxid,  indicating  alterations  in  chemical  com- 
binations which,  while  freeing  some  elements,  may  render 
others  more  resistant. 

The  striking  change  in  the  physical  properties  of  a 
residual  soil  from  the  parent  rock  depends  in  part  upon 
this  unequal  loss  of  elements.  As  a  rule,  unweathered 
residual  soils  are  highly  colored,  usually  red  or  yellow. 
This  results  from  the  accumulation  and  alteration  of 
the  iron.  Hence,  a  gray  limestone  will  produce  a  dark 
red  clay.  Other  properties,  as  the  texture,  result  in  the 
same  way.  Any  very  refractory  material,  as  chert  in 
limestone  or  quartz  in  igneous  or  secondary  rocks, 
is  likely  to  persist  and  remain  scattered  through  the 
soil.  The  cherty  hills  of  Tennessee  and  the  Ozarks  are 
examples  of  the  former,  and  the  topography  of  the  coun- 
try is  largely  determined  by  the  accumulation  of  this 
material.  Some  of  the  stony  soils  of  the  Piedmont 
regions  are  examples  of  the  second  type  of  soils.  The 


CUMULOSE  SOILS  41 

occurrences  of  these  refractory  materials  in  layers  may 
exercise  a  very  unfavorable  effect  on  the  agricultural 
value  of  such  land. 

Further,  residual  soils  are  seldom  uniform  in  texture. 
The  clays  are  usually  gritty,  especially  when  derived 
from  igneous  rocks.  It  has  been  suggested  that  this  is 
due  to  the  accumulation  of  silica  set  free  from  the  silicic 
minerals  in  their  loss  of  alkaline  materials.  In  this  state 
much  of  it  passes  into  solution  and  is  removed,  which 
probably  explains  some  of  the  large  losses  of  this  ele- 
ment. But,  where  the  decay  is  rapid,  not  all  of  the 
silica  can  be  so  removed,  and  it  combines  with  oxygen, 
to  form  quartz  particles. 

All  of  these  considerations  should  be  kept  in  mind 
in  the  study  of  residual  soils,  as  they  assist  in  under- 
standing their  characteristics. 

13.  Cumulose  soils.  —  Cumulose  soils  consist  of 
years  and  even  centuries  of  accumulations  of  plant 
remains.  They  occur  in  every  section  of  the  coun- 
try in  areas  of  from  a  fraction  of  an  acre  to  thou- 
sands of  acres,  known  as  peat  bogs  and  muck  swamps. 
The  one  condition  which  always  accompanies  these 
deposits,  and  is  most  largely  responsible  for  their 
existence,  is  poor  drainage.  Such  a  condition  may 
result  from  a  variety  of  circumstances.  In  the  North 
Central  states  of  the  glacial  section,  scattered  over  the 
undulating  country,  are  numerous  small  depressions 
where  water  accumulates  during  much  of  the  year, 
together  with  a  small  amount  of  sediment  from  the 
surrounding  hills.  These  conditions  favor  the  large 
growth  of  vegetation  which,  upon  its  death,  is  slowly 


CUMULOSE  SOILS  43 

accumulated  on  the  bottom  of  the  depression.  The 
dead  remains  are  kept  saturated  with  water,  which 
excludes  the  air  and  keeps  down  the  temperature,  and 
otherwise  hinders  decay,  so  that  the  annual  additions 
exceed  the  annual  loss  by  decay.  Hence,  an  accumu- 
lation of  vegetable  remains  is  inevitable.  This  is  the 
genesis  of  hundreds  of  the  mucky  marshes  throughout 
the  country.  Old  abandoned  stream  channels  are  a 
common  beginning  of  such  accumulations.  Very  similar 
in  origin  are  muck  and  peat  beds,  which  were  formerly 
deep  lakes.  A  peculiarity  of  fresh  water  deposits  of  this 
sort  are  beds  of  marl,  or  impure  lime  carbonate,  beneath 
the  vegetable  matter. 

A  slightly  different  type  of  these  deposits  are  the 
seacoast  swamps  from  Massachusetts  to  Texas,  many  of 
which  are  of  large  extent.  These  have  formed  in  brackish 
water 

The  chemical  composition  typical  of  many  of  the 
cumulose  deposits  is  shown  in  the  accompanying  table. 
The  physical  and  chemical  properties  of  such  soil  will 
be  more  fully  discussed  under  the  head  of  physical 
properties  of  organic  soils. 

Cumulose  deposits  are  characterized  chemically  by 
their  large  percentage  of  carbonaceous  matter.  If  the 
vegetation  suffered  no  decay  and  received  no  mineral 
matter,  it  would  be  simply  a  mass  of  plant  tissue;  but, 
as  has  been  stated,  there  is  every  degree  of  "wash" 
mixed  with  the  dead  plants.  These  also  have  accumu- 
lated to  all  depths  from  almost  nothing  to  many  feet  in 
thickness.  Many  areas  of  soil,  such  as  Miami  black  clay 
and  the  Clyde  soils  of  the  northern  states,  and  the 


44 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


TABLE   IV 
CHEMICAL  COMPOSITION  OF  CUMULOSE  DEPOSITS 


I 

II 

III 

IV 

V 

VI 

VII 

£  ' 

«! 

*o 

SJ-* 

VI 

Florida  Bulletin  14, 
Average  8  samples 

Merrill,  p.    315. 
Swamp 
North  River 
Carteret  Co., 
N.  Carolina 

Illinois  Bulletin  123 

is 

is 

ll 

|.s 

P 

ch 

si 

'8s 

£Z 
S1 

Center  of 
swamp 

1    Moisture 

49.00 
30.00 

17.00 

2.50 
'7.70 

9.60 
87.25 

2.  Volatile  matter    .... 
3.  Organic  matter     .  .  . 

71.80 

84.72 
42.36 

86.70 
43.35 

78.52 
39.62 

4.  Insoluble  matter  .  .  . 
5.  Soluble  silica  (SiO,) 
6.  Insoluble  silica  (SiO2. 
7.  Ash 

18.47 

3.70 

80.84 

1.52 

2000 

8.  Iron  oxide  (Fe2O3).  .  . 
9.  Alumnia  (A12O3)    .  .  . 
10    Lime  (CaO) 

2.60 
1.38 
0.51 

1.18 
2.69 
0.44 

0.15 
0.39 
0.36 

11.  Carbon  dioxid  (COS) 
12.  Magnesia  (MgO)    .  .  . 
13.  Soda  (Na,O)  

1.77 
0.10 
0.20 

0.22 
0.02 
0.07 
0.08 

6.14 
0.13 
0.06 
0.06 

14.  Potash  (KjO)  

0.32 
0.32 
0.62 
1.06 

0.14 
0.10 
1.11 

0.318 
0.196 
3.48 

0.36 
0.15 
3.36 

0.366 
0.12 
3.14 

15.  Phosphoric  acid  P2O5 
16.  Nitrogen  (N)   

17.  Sulphuric  acid  (SO,)  . 

0.06 

Portsmouth  and  other  soils  of  the  southern  states, 
represent  the  very  first  stages  in  the  formulation  of 
such  cumulose  deposits.  That  is,  they  are  simply 
mineral  soils  with  a  high  content  of  organic  matter. 
14.  Transported  soils. — The  four  great  agencies  of 
soil-transportation  are  (1)  water,  (2)  ice,  (3)  wind,  and 


TRANSPORTED   SOILS  45 

(4)  gravity.  It  will  be  remembered  that  each  of  these 
agencies  was  mentioned  as  active  in  soil -formation 
through  the  physical  and  chemical  forces  brought  to 
bear  on  rocks.  The  material  is  moved  from  its  original 
position  and  laid  down  under  new  conditions  which 
develop  properties  entirely  different  from  those  pos- 
sessed by  sedentary  soils. 

Of  the  four  groups,  those  soils  transported  by  water 
are  easily  the  most  extensive,  and  next  to  these  in  area 
stand  those  moved  by  glacial  action.  Wind-moved 
soils  are  of  much  importance  in  some  sections,  but 
gravity-moved  soils  are  of  small  extent. 

15.  Gravity  or  colluvial    soils. — In    mountainous  or 
hilly  regions,  soil  material  of  all  dimensions  is  moved 
down  the  slope  under  the  pull   of  gravity.     In  those 
sections  of  the  country  where  stone  fences  are  common, 
the  accumulation  of  soil  on  the  uphill  side  of  the  fence, 
due  to  gravity  movement,  not  infrequently  reach  the 
top  of  the  wall.    Because  of  its  associations  with  a  hill 
(Collis  meaning  hill),  such  material  is  termed  colluvial. 
The  first  footings  of  soil  in  the  niches  and  at  the  base 
of  a  rocky  ledge  are  usually  of  this  sort,  and  in  mountain 
regions  the  accumulation  of  such  material  is  sometimes 
large. 

16.  Water. — It    has   been  shown  how  water  is  able 
to    transport    sand    and    even    boulders    several    times 
heavier  than  itself,   if  it   be   flowing  with   a  sufficient 
velocity.    (See  page  24.)    This  large  transporting  power 
may  be  observed  in  any  creek  or  rivulet,  and  in  every 
hilly  region  it  is  brought  forcibly  to  the  farmer's  notice 
in  the  gullies  formed  by  heavy  rains.    The  bed  of  every 


46  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

stream  is  strewn  with  material  which  has  been  dropped 
by  the  water.  If  the  bed  of  the  stream  is  steep,  it  is 
paved  almost  entirely  with  large  stones  and  boulders. 
If  the  bed  is  very  flat  and  the  flow  slow,  the  bottom  is 
formed  by  sand  or  silt.  These  variations  are  well  illus- 
trated by  the  ripples  and  quiet  pools  of  almost  any 
stream,  the  former  being  stony,  the  latter  more  fine- 
textured.  This  principle  of  the  varying  carrying  power 
of  flowing  water  is  of  great  agricultural  importance. 
It  results  in  sorting  the  material  which  comes  into  the 
water,  and  the  particles  of  one  size  are  deposited  to- 
gether. In  this  way  is  accumulated  a  fine  pure  clay 
in  one  place,  a  sand  at  another  place,  and  gravel  at 
still  another.  These  formations  are  strikingly  different 
in  their  relations  to  plant-growth  because  of  their  dif- 
ferent physical  and  chemical  properties,  as  will  be  shown 
in  the  further  discussion  of  these  matters. 

The  character  of  such  soil  depends  upon  two  factors: 

(1)  The  character  of  the  rocks  from  which  it  is  derived. 

(2)  The  conditions  under  which  it  is  deposited. 

The  soils  of  this  group  are  by  far  the  most  important, 
agriculturally,  of  any  which  will  be  discussed,  on  account 
of  both  their-  relative  area  and  crop  relations.  In  a 
general  way,  they  may  be  divided  into  three  sub-groups; 
but  it  is  impossible  to  draw  any  sharp  line  of  distinction 
between  these  groups.  These  are:  (1)  Marine  soils. 
(2)  Lake  and  pond  deposits,  or  lacustrine  soils.  (3) 
Stream-laid,  or  alluvial  soils. 

17.  Marine  soils. — The  marine  soils  occupy  large 
areas  in  the  United  States  and  many  other  countries. 
They  consist  of  stratified  gravels,  sands,  silts  and  clays 


SOILS    DEPOSITED    BY    WATER  47 

deposited  in  shallow  off-shore  water,  and  subsequently 
raised  above  sea-level,  where  they  have  been  subject 
to  erosion  by  the  present  drainage  channels,  so  that 
they  are  furrowed  by  a  ramifying  system  of  shallow, 
steep-sided  gorges.  These  channels  reveal  the  different 
sorts  of  material  from  coarse  to  fine,  and  have  exposed 
each  of  them  over  considerable  areas.  The  material 
has  not  been  deposited  long  enough  or  buried  deep 
enough  to  be  much  consolidated,  although  there  are 
very  soft  shales  and  limestones  in  the  Gulf  states  which 
are  only  partially  consolidated. 

18.  Lacustrine  soils. — Closely  related  to  the  marine 
soils  are  soils  deposited  in  lakes,  suc'h  as  those  fringing 
the  Great  Lakes.    These  lacustrine  soils  differ  from  the 
former  in   the   different   source   of   their   material    and 
somewhat  different  conditions  of  deposition.    Most  of 
them   are  fresh-water   bodies,   but   in   some   instances, 
as  Great  Salt  Lake,  they  are  brackish.    It  is  impossible 
to  draw  any  definite  line  of  distinction  between  these 
two  sub-groups  of  soils  further  than  in  the  extent  and 
character  of  the  waters  in  which  they  were  deposited, 
and  for  a  specific  understanding  of  their  characteristics 
the  respective  types  must  be  studied  in  detail. 

19.  Alluvial  soils. — Along  every  stream  course  is  a 
ribbon  of  material  formed  by  the  deposition  from  the 
water  of  that  stream  at  either  normal  or  flood  time. 
Along  the  steep-bedded  streams  it  is  very  narrow  and 
usually  coarse,  often  with  a  base  of  stone  covered  by  a 
veneer  of  fine   material.     As   the   course   becomes   less 
steep,  it  widens  and  is  more  meandering.    The  stream 
swings  from  side  to  side  of  its  valley  in  large  sweeping 


48 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


curves,  which  become  actually  tortuous  in  very  flat 
bottoms.  Such  a  crooked  channel  is  much  reduced 
in  capacity  over  a  straight  channel,  and  therefore  in 
flood  season  the  water  is  piled  up  over  the  bank  and 


Fid.  15.    Shows  the  stratified  arrangement  of  a  gravelly  soil 

overflows  the  adjacent  land.  When  the  water  passes 
from  the  deep  channel,  its  velocity  is  checked,  and  some 
of  the  material — the  coarsest — is  deposited  on  the 
bank.  The  finer  material  is  carried  further  out  If  there 
is  a  general  movement  over  the  whole  bottom  the 
very  finest  material  may  not  be  deposited,  and  con- 


CHEMICAL  COMPOSITION  OF   MARINE  SOILS 


49 


TABLE   V 

CHEMICAL  COMPOSITION   OF   SOILS   DEPOSITED   BY   WATER. 
COMPLETE    ANALYSES 

COASTAL  PLAIN  SOILS  OF  MARYLAND 


I 

II 

III 

IV 

V 

Light  sandy  loam.  Truck 
soil.  Columbia  formation. 
Maryland.  Average  of  five 
samples. 

Heavy,  nne  sandy  loam. 
Tobacco  soil.  Chesapeake 
formation.  Maryland. 
Average  of  four  samples. 

Clay  loam.  Corn  and 
wheat  soil.  Columbia  for- 
mation. Maryland.  Aver- 
age of  three  samples. 

Clay.  Very  low  produc- 
tivity. Potomac  formation. 
Maryland.  Average  of 
three  samples. 

Sand,  "Pine  Barrens." 
Lafayetteformation.  Mary- 
land. 

1.  Insoluble  

2.  Silica  (SiO,)  

92.30 

83.86 

80.55 

6426 

94.32 

3.  Alumina  (Al,Oj)     

3.20 

6.10 

8.82 

1992 

2.66 

4.  Ferric  iron  (Fe,O,)   

0.91 

2.63 

2.67 

5.74 

1.25 

5.  Ferrous  iron  (FeO)  

6.  Sulfur  trioxid  (SO3)    

0.08 

0.12 

0.07 

0.09 

7.  Phosphoric  acid  (P,O5)  .  .  . 
8.  Lime  (CaO)  

0.05 
0.41 

0.23 
050 

0.42 
0.47 

0.16 
044 

0.02 
0.04 

9.  Carbon  dioxid  (CO,)       .    . 

0.08 

006 

005 

0  15 

10.  Magnesia  (MgO)  

0.35 

0.45 

0.29 

0.59 

0.07 

11.  Soda  (NaJO)  

0.50 

0.56 

0.49 

0.58 

0.11 

12.  Potash  (K,O)  

0.70 

0.92 

1.22 

1.50 

0.12 

13.  Water  

0.23 

1.30 

1.28 

0.75 

14.  Organic  matter  

15.  Volatile  matter   

1.13 

300 

3.26 

6  58 

1.21 

sequently  the  accumulated  soil  is  a  fine,  friable  loam 
rather  than  a  clay.  Heavy  alluvial  clay  is  seldom  found 
outside  the  larger  river  bottoms  and  generally  in 
depressions  remote  from  the  channel. 

Another  source  of  heavy  soil  is  ponds  formed  by  the 
cutting  off  of  bends  in  the  channel.    These  "ox-bow 


50 


TABLE  V,  continued 

CHEMICAL  COMPOSITION   OF  SOILS   DEPOSITED   BY   WATER. 

COMPLETE  ANALYSES 
BRICK  CLAYS  OF  SOUTHERN  PLAIN 


VI 

VII 

VIII 

IX 

X 

XI 

5- 

h 

.20 
J'l 

«)   r 

i 

'    J 

•-'"z 

r? 

0 

•f 

V 

"i<5 

i 

|| 

-   ZL 

C  Q. 

1-3 
E| 

3f 

ji 

*j 

? 

fi'I 

a 

sfe 

±z 

•« 

o  6  * 

_3 

c.— 

5  - 

*i 

Is 

tLii 

>, 

£  B 

'5. 
>> 

0   X 

|i 

la 

WJI 

i 

i 

1  .  Insoluble  

+  silica 

93.23 

94.46 

2.  Silica  (SiO,)  

53.75 

70.45 

90.00 

44.40 

sol. 

1.67 

3.  Alumina  (A1,O3)  .  . 

24.91 

17.34 

4.60 

17.90 

2.36 

0.92 

4.  Ferric  iron  (Fe,O3) 

7.99 

3.16 

1.44 

4.50 

1.25 

0.32 

5.  Ferrous  iron  (FeO) 

6.  Sulfur  trioxid  (SO3 

• 

0.09 

7.  Phosphoric     acid 

(P,O4)  

0.03 

0.11 

8.  Lime  (CaO)   . 

070 

0.25 

0  10 

9  50 

0.12 

0.07 

9.  Carbon  dioxid  CO,. 

9.55 

0.02 

10.  Magnesia  (MgO)  .  . 

1.12 

0.22 

0.10 

1.88 

0.18 

0.04 

11.  Soda  (Na,O)  

•» 

•j 

^ 

0.07 

0.04 

12.  Potash  (K,O) 

V  2.94 

>  0.70 

< 

t 

0.26 

0.19 

13.  Water  

1.03 

0.98 

3.04 

4.58 

14.  Organic  matter.  .  . 

15.  Volatile  matter.  .  . 

6.63 

2.33 

1.88 

bends,"  "cut-offs,"  or  "bayous,"  as  they  are  variously 
termed,  become  in  succession  lakes,  ponds  and  marshes, 
where  the  clay-laden  water  is  gradually  evaporated 
or  filtered  away,  leaving  behind  only  the  very  fine 
material  that  may  be  carried  in  suspension  almost 
indefinitely.  As  a  result  of  these  processes  much  of  the 


CHEMICAL  COMPOSITION  OF  GLACIAL  LAKE  SOILS    51 


TABLE   V,  continued 

CHEMICAL  COMPOSITION   OF   SOILS   DEPOSITED   BY   WATER. 
COMPLETE  ANALYSES 

GLACIAL  AND  WESTERN  SOILS 


XII 

XIII 

XIV 

XV 

XVI 

Lacustrine  clay,  Albany, 
New  York 

£~o 
«.s 

11 

a  a 
it 

1* 

Adobe,  Salt  Lake  City, 
Utah 

Leila  Clay,  Lake  material, 
St.  Lawrence  Valley 

Clay.  Lake  material, 
Shore  Lake  Michigan,  Mil- 
waukee, Wisconsin. 

1.  Insoluble  

2.  Silica  (SiO,)  

5560 

66.69 

19.24 

56.17 

40.22 

3.  Alumina  (A1,O,)  

14  80 

14  16 

326 

24.25 

8.47 

4.  Ferric  iron  (FeaO3)  

580 

4  38 

1  09 

283 

5.  Ferrous  iron  (FeO)    

3.54 

0.48 

6.  Sulfur  trioxid  (SO3)  

0.41 

0.53 

0.13 

7.  Phosphoric  acid  (P,OS)  .  .  . 
8.  Lime  (CaO)  

0.15 
5  70 

0.29 
2  49 

0.23 
38.94 

209 

0.05 
15  65 

9.  Carbon  dioxid  (CO2)  

4.94 

0.77 

29.57 

18.76 

10.  Magnesia  (MgO)    

2.48 

1.28 

2.75 

2.57 

7.80 

11.  Soda  (Na,O)  

1.07 

0.67 

t 

2.25 

0.84 

12.  Potash  (K,O)  

3.23 

1.21 

t 

4.06 

2.36 

13.  Water  

5.18 

4.84 

1.67 

4.69 

1  .95 

14.  Organic  matter  

2.00 

2.96 

0.32 

15.  Volatile  matter  

soil  formed  in  stream  bottoms  is  friable  and  easily  tilled, 
but  they  also  give  rise  to  some  of  the  heaviest  and  most 
intractable  clay  soil  known.  Soils  of  this  sub-group  may 
be  either  very  uniform  or  exceedingly  variable  in  fine- 
ness. It  is  evident  from  what  has  been  said  that,  the 
smaller  the  stream,  the  more  variable  the  soil  is  likely  to 


52 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


TABLE    V,  continued 

CHEMICAL  COMPOSITION  OP  SOILS  DEPOSITED  BY  WATER. 
STRONG  HYDROCHLORIC  ACID  ANALYSES. 


XVII 

XVIII 

XIX 

XX 

XXI 

XXII 

XXIII 

fcT 

4) 

> 

6 

6 

1 

§ 

"o 

_o 

et 

= 

o 

> 

2 

J  E 

ij 

O 

CQ 

JS 

Q    gj 

3  O 

o 

S  - 

"8 

B3 

§  „ 

f  I 

*| 

.J3 

«  * 

js  * 

-  * 
a  * 

X 

.£•« 

•3 

EO 

-O 

g^ 

f 

EH 

.O 

l« 

^g 

0 

p 

•o 

~ 

•S 

^^j 

•3  1 

«, 

| 

i 

13 

> 

E 

m 

pq 

c8 

(* 

1.  Insoluble    

70.92 

44.23 

77.05 

83.22 

84.97 

81.77 

79.99 

2.  Silica  (SiO,)  

*5.65 

7.68 

7.76 

3.  Alumina  (A1,O3)  
4.  Ferric  iron  (FejO,)  .  . 
5.  Ferrous  iron  (FeO) 

5.58) 
3.62  / 

{1.58 

>4.91 
/2.66 

2.11 
5.82 

1.42 
2.71 

3.33 
3.07 

2.78 
3.40 

6.  Sulfur  trioxid  (SO,)  . 

0.29 

0.15 

0.02 

0.15 

7.  Phosphoric  acid(P,O5 

0.34 

0.12 

0.18 

0.243 

0.04 

0.05 

0.06 

8.  Lime  (CaO)  

5.66 

23.98 

1.03 

0.56 

0.44 

0.32 

0.95 

9.  Carbon  dioxid  (CO,). 

4.00 

18.00 

0.81 

0.44 

10.  Magnesia  (MgO)  

1.85 

0.94 

0.93 

1.13 

0.16 

0.18 

0.21 

11.  Soda  (Na.O)    

0.23 

025 

0.96 

0.40 

0.48 

0.39 

0.31 

12.  Potash  (K,O)   

0.88 

0.22 

1.45 

0.48 

0.80 

1.21 

0.44 

13.  Water  

3.26 

2.42 

3  56 

1  32 

369 

2.18 

4.10 

14.  Organic  matter.  .  . 

062 

1.54 

0.51 

15.  Volatile  matter  

269 

734 

6  52 

4  60 

*  Soluble. 

be.  It  embraces  large  areas  of  the  most  productive  soils. 
Properly  drained,  bottom  lands  are  generally  regarded 
with  favor  for  several  of  the  staple  crops.  Corn  is  prob- 
ably the  most  grown.  Wheat  is  important  on  the  heaviest 
soils.  They  are  generally  rich  in  organic  matter  to  an 
unusual  depth  because  they  represent  largely  the  wash 


CHEMICAL  COMPOSITION   OF  ALLUVIAL  SOILS      53 


TABLE  V,  continued 

CHEMICAL  COMPOSITION  OF  SOILS  DEPOSITED  BY  WATER.    STRONG 
HYDIOCLILORIC  ACID  ANALYSES 


XXIV 

XXV 

XXVI 

XXVII 

XXVIII 

XXIX 

Clay  loam,  Marshall  Co., 
Minnesota.  Red  River 
Valley 

Lake  Clay.  Red  River  Val- 
ley, Crookston,  Minnesota 

4 
•^1 

§>•* 

Loam.  Red  River  Valley. 
Moorehead,  Minnesota 

Buckshot,  Clay.  Yasoo 
bottoms,  Mississippi 

Silt  loam,  Colorado  river, 
San  Diego  Co.,  California 

1.  Insoluble  

41.21 

39  17 

60.21 

45.06 

51.06 

58.57 

2.  Silica  (SiO,)  

837 

1509 

9.00 

1643 

20.70 

5.33 

3.  Alumina  (A1,O,)  .  . 
4.  Ferric  iron  (Fe,O,) 
5.  Ferrous  iron  (FeO) 

10.72 
3.48 

13.61 
3.98 

9.15 
3.94 

10.20 
4.22 

10.54 
5.82 

8.40 
4.14 

6.  Sulfur  trioxid 
(SO.)  

0  10 

006 

0  11 

009 

0.02 

0.15 

7.  Phosphoric  acid 

0.19 

0.28 

0.16 

0.27 

0.30 

0.13 

8.  Lime*  (CaO)  .  .    . 

7.45 

8.10 

1.07 

8.84 

1.35 

8.67 

9.  Carbon  dioxid  (CO,) 
10.  Magnesia  (MgO)  .  . 
1  1  .  Soda  (Na,O)  

14.26 
4.48 
0.48 

13.27 
2.04 
0.40 

0.13 
0.84 
0.61 

7.22 
3.02 
0.27 

i.67 
0.33 

7.82 
2.97 
0.16 

12.  Potash  (K,O) 
13.  Water  

0.25 

0.60 

0.90 

0.81 

1.10 

1.18 

14.  Organic  matter  .  . 

0.89 

0.81 

5.16 

15.  Volatile  matter.  .  . 

6.22 

3.20 

14.29 

2.61 

7.37 

3.34 

from  the  surface  layer  of  the  upland  soils,  and  they  are 
not  old  enough  to  have  lost  this  supply  of  organic  matter 
by  decay.  Frequently,  the  supply  is  replenished  by 
annual  additions. 

Table   V   illustrates   the   variations   in    the    propor- 
tion of    the    different    elements    in    water    deposits   of 


54  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

different  physical  properties,  from  different  parts  of 
the  United  States.  Many  of  these  analyses  are  less 
complete  with  reference  to  some  of  the  plant-food  con- 
stituents than  is  desirable  for  the  purpose  here  intended. 
So  far  as  -possible,  analyses  of  the  entire  soil  have  been 
used,  but,  where  these  could  not  be  obtained,  analyses 
of  the  strong  hydrochloric  acid  extract  are  given. 

20.  Ice — glacial  soils. — In  many  parts  of  the  world 
there  exist  soils  which  have  been  formed  under  the 
influence  of  large  bodies  of  ice. 

In  earlier  times,  masses  of  ice  extended  far  to  the 
southward  over  the  country  now  devoted  to  agricul- 
tural purposes.  Around  the  world  this  mass  of  ice 
appears  to  have  extended  down  from  the  north  and 
south  poles  to  a  zig-zag  limit.  It  reached  into  Asia, 
Central  Europe  and  the  American  Continent  as  far 
south  as  New  York  City,  Cincinnati,  St.  Louis,  Kansas 
City  and  Omaha,  and  farther  west  in  the  Puget  Sound 
region  it  extended  south  across  the  Columbia  river. 
All  the  country  north  of  this  line  with  the  exception 
of  one  or  two  small  areas  was  covered  by  an  -immense 
sheet  of  ice  which  moved  slowly  down  from  the  north- 
ward. In  the  southern  hemispheres  are  similar — though 
more  limited — traces  of  the  same  condition. 

The  depth  of  the  ice  was  so  great  that  it  flowed 
over  such  elevations  as  Mount  Washington  in  New 
Hampshire  and  over  the  Adirondacks  in  New  York. 
Its  general  movement  in  the  northern  hemisphere  was 
southward.  Its  flow  was  modified  by  the  original  topog- 
raphy of  the  country,  but  its  depth  was  so  great  it 
was  able  to  disregard  and  override  many  of  the  land 


GLACIAL  SOILS  55 

forms.  It  advanced  first  through  the  valleys,  and  at  the 
bottom  of  the  mass  appears  to  have  been  guided  in 
its  flow  by  these  channels.  The  advance  probably 
consumed  a  long  period  of  years,  or  even  centuries, 
and  the  retreat  was  similarly  slow.  Along  the  margin, 
as  in  modern  glaciers,  there  were  annual  fluctuations 
in  the  position  of  the  ice  front  which  are  indicated 
by  the  greater  or  less  accumulation  of  rock  debris,  as 
undulating  piles  of  earth  or  terminal  moraines.  This 
ice  picked  up  immense  amounts  of  material  along  its 
way.  Most  of  the  original  soil  overlying  the  rocks  was 
swept  away.  Prominences  were  torn  away  or  planed 
down,  and  depressions  were  filled  up.  Masses  of  rock 
were  ground  to  powder,  and  boulders  were  transported 
to  entirely  new  surroundings:  The  advance  of  the  ice 
over  the  country  largely  disregarded  the  rock  forma- 
tions, as  it  did  topographic  forms,  so  that  the  rocks 
and  soil  materials  from  many  sources  were  mixed  and 
ground  together.  In  this  way,  the  granite  boulders 
strewn  over  the  surface  near  the  southern  margin  of 
the  ice  extension  in  the  United  States  were  derived 
from  points  hundreds  of  miles  to  the  northward,  even 
into  northern  Canada.  The  movement  was  not  straight 
south,  but  deflected  by  broad  obstructions  in  the  land, 
so  that  the  source  of  the  soil  in  any  region  is  determined 
by  the  direction  of  movement  in  that  section.  This 
movement  may  often  be  traced  by  the  kind  of  rocks 
which  have  been  left,  and  may  lead  back  to  the  ledges 
from  which  they  were  derived. 

The  relation  of  glacial  soils  to  the  underlying  rock 
depends    entirely    on    the    conditions    which    prevailed 


56  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

in  that  region  when  it  was  formed.  In  central  Michigan, 
the  soil  bears  scarcely  any  relation  to  the  underlying 
rock  of  the  region  ;  but,  in  Southern  New  York  and 
Northern  Pennsylvania,  the  very  shaley  character  of 
the  soil  may  be  traced  to  the  broad  area  of  shale  rock 
which  underlies  all  that  section  of  country,  and  which 
was  the  main  source  of  the  glacial  debris.  As  one  passes 
northward  through  the  finger-lake  region  of  New  York, 
the  proportion  of  limestone  and  other  foreign  material 
resting  on  the  gray  shale  increases  until  the  exposures 
of  ledge  limestone  are  met  at  Syracuse  and  Rochester, 
portions  of  which  rock  had  been  raked  far  southward 
by  the  ice-movement.  This  shifting  and  mingling 
of  material  must  always  be  kept  in  mind  in  examining 
glacial  soils. 

Purely  glacial  deposits  differ  in  chemical  and  physical 
properties  from  soils  derived  from  the  same  formations 
by  other  means.  There  is  a  large  element  of  mechanical 
grinding  without  any  large  amount  of  chemical  change 
or  solution.  The  particles  have  not  been  subjected  to 
long-continued  leaching,  which  characterizes  residual  or 
marine  soils.  Such  material  is  chiefly  rock-flour,  that  is, 
pulverized  rock.  The  readily  soluble  minerals  and  ele- 
ments are  therefore  present  in  proportionately  larger 
amounts  than  in  soil  formed  by  other  means.  While  a 
residual  soil  from  limestone  may  be  very  poor  in  lime 
carbonate,  a  glacial  soil  formed  from  lime-rock  is  often 
rich  in  lime,  sometimes  containing  50  per  cent  of  that 
constituent,  as  has  been  found  in  some  Dakota  soils.  As 
appears  from  the  tables  of  analyses,  such  soils  are  gen- 
erally rich  in  all  of  the  basic  elements. 


CHEMICAL  COMPOSITION  OF  GLACIAL  SOILS        57 


TABLE   VI 

CHEMICAL  COMPOSITION  OF  GLACIAL  SOILS 
HYDROCHLORIC  ACID  ANALYSES 


I 

II 

III 

IV 

V          VI 

o 

si 

i! 

02  .0 

Clay  loam, 
Strongville,  Ohio 

Loam, 
Columbus,  Ohio 

Clay  loam, 
Germantown,  Ohio 

Loam  subsoil, 
Prairie, 
Western  Minn. 

£ 

1 

1.  Insoluble  

87.85 

83.80 

83.87 

89.20 

73.95 
6.85 
4.63 
3.05 

74.05 
S,t() 
3.27 
5.44 

2.  Silica  (SiO,)  

3.  Alumnia  (A1,O,)  
4.  Ferric  iron  (Fe,O,).  .  . 
5.  Ferrous  iron  (FeO)  .  . 

3.46 
3.30 

4.11 
4.72 

4.26 
3.63 

3.69 
2.26 

6.  Sulfur  trioxide  (SO,)  . 
7.  Phosphoric  acid(P,O5) 
8.  Lime  (CaO)  

0.04 
0.11 
0.25 

0.03 
0.09 
0.18 

0.10 
0.15 
0.69 

0.03 
0.12 
0.13 

0.04 
0.26 
0.70 
0.36 
0.36 
0.42 
0.40 

0.12 
0.16 
0.51 
0.09 
0.22 
0.16 
0.22 

9.  Carbon  dioxid  (CO,)  . 
10.  Magnesia  (MgO)  
11.  Soda  (Na,O)  

0.39 
0.34 
0.25 

0.45 
0.29 
0.22 

0.62 
0.78 
0.56 

0.37 
0.23 
0.21 

12.  Potash  (K,O)  

13.  Water  

14.  Organic  matter  

15.  Volatile  matter  

4.09 

5.92 

5.64 

3.86 

9.12 

7.29 

The  physical  properties  of  glacial  soils  are  also  dis- 
tinctive. Excepting  subsequent  modifications  due  to 
water,  such  deposits  show  little  or  no  stratification  or 
sorting.  They  are  heterogeneous  in  material  and  arrange- 
ment. Much  of  such  material  is  termed  boulder  clay, 
from  the  mixture  of  coarse  and  fine  particles.  It  is 
also  to  be  noted  that  such  soils  contain,  relatively,  a 
larger  proportion  of  silt  particles,  and  a  smaller  amount 


58 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


Flo   16.    Section  of  glacial  soil,  showing  its  uneven  texture  and  dense  structure. 
When  unmodified  by  water  action,  it  usually  shows  no  stratification 

of  clay,  than  soil  formed  by  purely  chemical  process 
from  the  same  rock. 

Associated  with  the  results  of  pure  ice-action  is 
much  modified  glacial  till,  due  to  the  influence  of  great 
volumes  of  water.  Naturally,  the  melting  of  the  ice 
results  in  immense  volumes  of  water,  which  drain 
away  over,  under,  or  along  the  ice  margin.  Temporary 
streams  of  large  size  and  great  violence  existed 


GLACIAL  SOILS  59 

and  there  were  also  ponds  and  lakes,  some  of  the  latter 
of  very  large  extent.  This  water  further  assisted  in 
moving  the  ice  debris.  Such  deposits  are  called  modi- 
fied drift,  or  aqueo-glacial  deposits.  For  this  reason, 
they  have  in  part  been  included  with  glacial  soils. 
The  streams,  ponds  and  lakes  associated  with  the  ice 
have  given  rise  to  much  stratified  material,  and  these  de- 
posits are  intimately  related  in  many  ways  to  the  purely 
ice  deposits.  Beds  of  gravel,  sand  and  clay  are  frequently 
found,  and  so  intimate  is  their  relation  to  the  purely 
ice  deposits  that  they  are  sometimes,  though  incor- 
rectly, classed  with  them.  These  deposits  of  modified 
till  generally  rest  upon  the  distinctly  ice  deposits,  and 
are  of  large  extent.  Around  the  Great  Lakes  and  in  the 
large  valleys  of  New  York  and  New  England,  in  the 
valley  of  the  Red  River  of  the  North,  and  in  many 
other  places  in  the  Central  States,  are  large  areas  of 
such  stratified  glacial  material,  ranging  in  fineness 
from  heavy  clay  to  coarse  gravel.  These  materials 
constitute  some  of  the  most  valuable  agricultural 
lands  of  the  country.  The  Great  Lakes  region  is  notably 
productive,  and  the  Red  River  Valley  of  the  North 
is  celebrated  for  its  production  of  small  grains. 

The  thickness  of  glacial  deposits  varies  greatly. 
Pre-glacial  valleys  may  be  filled  in,  and  the  evidence 
of  their  presence  completely  obliterated.  In  general,  the 
topographic  effect  of  glacial  action  is  to  level  the  surface. 
However,  in  the  New  England  states,  where  the  country 
is  very  mountainous,  the  rocks  very  hard  and  the  pre- 
glacial  soil  blanket  meager,  the  present  soil  covering  is 
generally  thin  and  very  stony. 


60  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

Further  west,  where  the  country  is  less  rugged  and 
the  rocks  less  refractory,  the  soil  covering  is  of  greater 
depth  and  generally  less  stony.  In  the  states  of  the 
Mississippi  valley,  the  broad,  level  areas  of  excellent 
agricultural  soil  are  very  largely  the  result  of  these 
glacial  influences. 

21.  Wind  or  seolian  soils. — Attention  has  been  di- 
rected to  the  transporting  power  of  wind.  It  is  continu- 
ally picking  up  particles,  which  are  deposited  in  accord 
with  the  same  general  laws  which  govern  water  deposits. 
The  material  thus  carried,  often  to  great  heights,  is 
again  brought  to  the  surface  by  gravity.  These  particles 
are  frequently  accelerated  in  their  fall  by  rain  and 
snow.  Every  particle  of  fog,  of  rain  and  of  snow  has 
for  its  nucleus  a  particle  of  dust  around  which  con- 
densation began,  and  for  this  reason  the  atmosphere 
is  always  most  clear  after  precipitation.  Large  amounts 
of  material  are,  in  the  course  of  time,  brought  to  earth 
in  this  way. 

This  continual  deposition  from  the  atmosphere  is 
illustrated  by  the  layer  of  dust  that  quickly  accumu- 
lates in  any  unoccupied  building,  however  tightly  it 
may  be  closed. 

Besides  this  general  filtering  of  dust  particles  from 
the  atmosphere,  there  is  the  definite  drifting  of  soil 
by  wind,  of  which  sand-dunes  are  the  most  common 
illustration.  These  occur  in  many  parts  of  the  world. 
They  are  likely  to  be  developed  wherever  dry  sand  is 
exposed  to  the  wind. 

Related  to  these  modern  wind  deposits  are  immense 
areas  of  soil  of  great  agricultural  value,  the  origin  of 


AEOLIAN  SOILS,    LOESS  61 

which  is  not  clearly  understood,  but  which  appears  to 
owe  its  existence,  at  least  in  part,  to  wind  deposition. 
This  is  the  so-called  loess,  a  fine,  silty  soil  of  remarkable 
uniformity  in  physical  and  mineralogical  composition. 
It  covers  thousands  of  square  miles  of  country  through- 
out the  Mississippi  valley  and  its  tributaries,  from  Cin- 
cinnati to  western  Nebraska,  and  from  west-central 
Wisconsin  to  southern  Mississippi.  It  lies  uncomformably 
over  formations  of  many  ages,  as  a  mantle  of  soft  earth 
of  varying  thickness.  It  does  not  extend  over  the  whole 
of  the  region  mentioned,  but  alternates  with  other 
formations,  especially  drift.  It  imparts  to  the  regions 
on  which  it  rests  a  soil  character  greatly  different  from 
what  would  exist  were  it  absent. 

Neither  is  it  limited  to  the  United  States,  for  it  occurs 
extensively  in  central  Europe,  where  it  extends  from 
northern  France  across  Belgium,  and  up  the  Rhine, 
Oder  and  Vistula  valleys  in  Germany;  and  into  central 
and  southern  Russia,  where  it  is  the  basis  of  the  famous 
"black  earth,"  or  tschernosem.  In  northern  China,  von 
Richtofen  has  described  it  as  covering  a  large  part  of 
the  region  drained  by  the  Hoang-Ho,  where  it  reaches 
a  thickness  of  1,000  feet. 

In  thickness  it  varies  greatly.  Over  much  of  the 
United  States  it  is  only  a  few  feet  in  thickness,  generally 
thinning  toward  the  outer  margin.  In  the  central 
areas  it  may  be  150  to  200  feet  in  thickness,  and,  simi- 
larly, in  other  countries  it  is  of  variable  thickness, 
reaching  the  great  depth  mentioned  above  for  China. 

A  striking  physical  character  of  the  loess  is  its 
ability  to  stand  for  a  long  time  in  vertical  cliffs,  although 


62  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

so  soft  it  may  be  easily  carved  with  a  shovel.  Another 
character  common  to  much  of  the  formation  is  the 
presence  of  nodules  and  tubes  formed  by  cementation 
by  lime  carbonate. 

The  loess  is  associated  in  occurrence  with  the  margin 
of  the  glacial  deposits,  especially  in  America  and  Europe, 
and  possibly  in  China.  Just  what  this  relation  is  is  not 
known,  but  much  of  the  loess  seems  to  be  a  fine  rock- 
flour  of  glacial  origin,  which  has  been  drifted  by  the 
wind  and  deposited  on  both  purely  glacial  deposits 
and  on  residual  and  water  deposits,  for  it  extends  from 
Illinois  southward  over  the  limestone  region  on  to  the 
coastal  plain  in  Mississippi. 

The  adobe  soils  of  the  arid  regions  are  thought  by 
some  to  be  related  to  the  loess  in  mode  of  formation. 
Adobe  also  has  peculiar  physical  properties,  later  to  be 
mentioned,  but  it  exhibits  a  closer  relation  to  water 
deposits  with  which  it  has  been  classed. 

In  parts  of  Kansas,  Nebraska  and  other  western 
states,  are  soils  formed  of  dust  from  volcanic  vents  and 
deposited  from  the  atmosphere.  Such  dust  may  be  so 
fine  as  to  be  carried  long  distances  and  remain  in  sus- 
pension for  a  long  period.  Dust  from  the  eruption  of 
Krokatoa,  in  the  island  of  Java,  was  wafted  around 
the  world,  and  gave  a  red  glow  to  the  sunset  for  a  year 
after  its  discharge. 

Table  VII  shows  the  chemical  composition  of  the 
wind  deposits,  chiefly  loess.  Columns  I  and  X  are 
analyses  of  the  hydrochloric  acid  solution.  All  others 
are  complete  analyses.  Agriculturally,  sand-dunes  are 
of  small  value,  largely  because  of  their  unfavorable 


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(63) 


64  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

physical  properties.  They  are  also  likely  to  be  highly 
silicious.  But  the  loess  formations  are  of  great  agricul- 
tural importance,  and  in  this  country  they  constitute 
some  of  the  most  important  soil  types.  In  some  sections 
its  value  has  been  greatly  reduced  by  erosion.  Some  of 
the  bluff  areas  along  the  Mississippi  river  are  thus 
modified,  and  some  of  the  loess  of  China  is  also  deeply 
eroded. 

But  the  physical  properties,  as  well  as  the  chemical 
properties  of  loess,  combine  to  give  it  in  general  a 
high  agricultural  value. 

VI.     HUMID    AND    ARID    SOILS 

• 

In  discussing  the  process  by  which  soil  is  derived 
irom  rock,  attention  was  directed  to  the  fact  that  phys- 
ical disintegration  results  in  material  having  different 
properties  from  those  derived  through  chemical  decom- 
position, and  that  the  relative  prominence  of  these 
two  processes  is  dependent  largely  on  climate.  Aridity 
is  one  of  those  phases  of  climate  which  markedly  alters 
the  balance  between  these  two  processes,  giving  the 
larger  ascendancy  to  the  physical.  Soils  formed  under 
arid  conditions  are  less  fine  in  texture  than  those  formed 
from  the  same  rock  in  humid  regions.  A  study  of  soils 
in  the  two  regions  reveals  a  much  greater  prevalence  of 
the  coarser  soils — the  sandy  and  loamy  soils — in  the 
arid  region. 

But  chemical  processes  are  not  absent,  for  in  every 
arid  region  there  is  some  precipitation  which  is  able  to 
bring  about  changes  in  the  minerals,  although  the 


CHEMICAL  COMPOSITION  OF  ARID  AND  HUMID  SOILS  65 


products  of  these  chemical  changes  are  likely  to  accu- 
mulate in  the  soil  because  of  the  absence  of  sufficient 
moisture  to  leach  them  away.  (See  page  307.)  Their 
presence  is  evidenced  by  incrustations  on  the  particles 
either  at  the  surface  or  in  the  mass  of  the  soil.  For  this 
reason,  the  unfavorable  conditions  which  would  tend 
to  result  from  the  coarser  grade  of  the  material  is  more 
than  offset  by  the  large  amounts  of  readily  soluble 
elements  present.  These  differences  are  well  illustrated 
by  the  following  table,  compiled  by  Hilgard  from  the 
results  of  many  acid  analyses  in  the  two  regions.  All 
soils  derived  from  limestone  are  excluded. 

TABLE   VIII 

CHEMICAL  COMPOSITION  OF  ARID  AND  HUMID  SOILS 
STRONG  HYDROCHLORIC  ACID  ANALYSES 


I 

11 

III 

Humid  regions. 
Average  of 
696  samples 

Semi-arid  re- 
gions.  Average 
of  178 
samples 

Arid  regions. 
Average  of 
573  samples 

1.  Insoluble  residue  . 

84.17 

75.04 

69.16 

2.  Soluble  silica  (SK)2).  .  . 
3.  Alumnia  (A1,O3)  
4.  Ferric  iron  (Fe3O.,).  .  .  . 
5.  Sulfur  trioxid  (SO.,)  .  .  . 
6.  Manganese  (MnO,)  .  .  . 
7.  Phosphoric  acid  (PjOj)- 
8.  Lime  (CaO)  

4.04 
3.66 
3.88 
0.05 
0.13 
0.12 
0.13 

8.46 
4.57 
2.08 
0.02 

0.21 
0  70 

6.71 

7.21 
5.48 
0.06 
0.11 
0.16 
1  43 

9.  Magnesia  (Mg()).. 

0  2\t 

0  47 

1  27 

10.  Soda  (NtuO)    .  . 

0  14 

0  32 

035 

11.  Potash  (K3O)  .. 

0  21 

0  33 

067 

12.  Humus  . 

1  .22 

3  24 

1  13 

13.  Water     and     organic 
matter.  . 

4  40 

8  55 

5  15 

66  THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

From  this  table  it  appears  that,  in  spite  of  the  finer 
texture,  the  humid  soils  contain  15  per  cent  less  soluble 
material  and,  as  compared  with  the  semi-arid  region, 
9  per  cent  less  soluble  material. 

VII.     RKSUME    OF    SCHEME     OF    CLASSIFICATION    AND    GEN- 
ERAL  CHARACTERISTICS    OF   THE    GROUPS 

From  the  foregoing  discussion  it  appears  that  each 
group  of  materials  may  have  properties  which  are 
fairly  characteristic.  Physically,  the  sedentary  materials 
differ  from  the  transported  material  chiefly  in  arrange- 
ment. In  the  transported  soils  those  laid  down  by 
wind  and  water  are  distinctly  stratified — that  is,  ar- 
ranged in  layers.  This  is  the  result  of  settling  or  sedi- 
mentation from  a  fluid,  and  such  soils  are  frequently 
spoken  of  as  sedimentary.  Wind  and  water  are  the 
only  two  media  in  which  sedimentation  occurs  in  nature, 
and  therefore  this  arrangement  indicates  their  influence. 
Thereby  the  extent  and  variation  of  such  deposits  mayt 
be  largely  interpreted. 

Upon  the  basis  of  these  formative  differences,  it  is 
possible  not  only  to  identify  the  different  soil  materials 
but  to  represent  their  extent  upon  maps.  The  broadest 
separations  represented  by  sedentary  and  transported 
soils  may  be  termed  divisions.  Within  these  divisions 
are  sub-divisions,  according  to  the  agency  or  material 
involved.  These  are  termed  provinces,  that  is,  meaning 
the  region  or  province  where  a  certain  set  of  conditions 
prevailed.  For  example,  in  the  sedentary  division  are 
residual  soils  from  igneous  rocks  and  from  limestone 


SOIL  CLASSIFICATION  67 

rocks.  These  latter  constitute  soil  groups,  and,  simi- 
larly, in  the  transported  division  there  is  the  sub-division 
or  province  of  soils  deposited  in  water,  and  these  are 
further  sub-divided  into  those  formed  in  the  ocean, 
marine;  in  lakes,  lacustrine,  and  by  streams  alluvial, 
each  constituting  a  soil  group.  Within  the  soil  group 
the  first  division  is  the  soil  series,  based  upon  the  fine- 
ness of  the  material,  color,  drainage  and  other  properties, 
and  each  series  is  made  up  of  soil  types,  the  material 
in  each  one  being  practically  identical  in  all  respects. 
The  series  and  type  distinctions  will  be  better  under- 
stood after  a  consideration  of  the  physical  properties 
of  soil.  Maps  of  soils  based  upon  such  a  classification 
are  constructed  by  several  countries  and  institutions, 
the  most  extensive  being  the  United  States  department 
of  Agriculture.  These  maps  are  constructed  upon  dif- 
ferent scales,  but  one  inch  to  one  mile  is  the  most  com- 
mon. The  maps  are  accompanied  by  legends  and 
reports,  for  the  proper  explanation  of  the  conditions 
in  the  area  reported  upon. 

Chemically,  there  is  also  a  wide  variation  among  soil 
materials  in  the  total  amount  of  the  elements  present. 
It  might  be  expected  that  the  repeated  and  long-con- 
tinued mixing  of  materials  from  many  kinds  of  rock 
would  result  in  a  very  great  uniformity  in  all  soils. 
This  is  true  of  the  number  of  elements  present,  for  no 
important  element  is  absent  from  any  soil.  Rut  the 
amount  may  differ  greatly.  Aside  from  organic  soils 
(cumulose),  the  most  striking  differences  occur  in  sand 
soils.  While  the  average  analyses,  of  many  sandstones 
and  sand  soils  reveals  a  fair  amount  of  all  elements, 


70 


THE   PRINCIPLES  OF  SOIL   MANAGEMENT 


23.  Texture. — The  size  of   the   individual    particles 
in  a  soil  is  a  large  determining  factor  in  all  of  its  prop- 
erties.  The  term  texture  is  used  to  refer  to  the  size  of 

the  individual  par- 
ticles of  which  a  soil 
is  composed. 

In  shape  the 
particles  are  very 
irregular.  Being 
minerals  or  mineral 
aggregates,  they 
tend  to  have  the 
characteristic  lines 
and  faces  of  their 
species.  Ordinarily, 
however,  the  nu- 
merous forces  that 
have  been  at  work 
in  the  formation  of 
the  soil  have  rounded  or  broken  the  mineral  into  angu- 
lar, jagged  or  partially  smoothed  fragments.  The  relative 
number  of  particles  of  corresponding  sizes  varies  greatly 
in  different  soils,  some  being  composed  largely  of  coarse 
particles  while  others  are  made  up  largely  of  fine  ones, 
^^he  relative  proportion  of  these  various -sized  particles 
influences  greatly  the  physical  properties  of  the  soil. 

24.  Textural  classification. — When  a  soil  is  divided 
into  groups  of  particles  of  approximately  one  size,  the 
process    constitutes    a    mechanical    analysis    and    each 
group  is  a  soil  separate.    The  limit  in  size  of  each  of 
these  groups  is  arbitrarily  arranged,  and  is  determined 


Fio.  17.  Fine  sand,  photomicrograph.  Magni- 
fied about  110  diameters.  Differences  in  color 
indicate  differences  in  mineral  composition.  Each 
particle  composed  of$ne  mineral. 


SOIL   TEXTURE 


71 


by  the  relative  value  of  the  different  sizes  in  determin- 
ing the  properties  of  the  soil  and  its  crop-producing 
power.  It  is  found  that  the  fine  groups  exert  relatively 
much  more  in- 
fluence, weight  for 
weight,  than  the 
coarse  ones.  There- 
fore there  are  more 
divisions  made 
amongthe  finethan 
among  the  coarse 
particles. 

26.  Textiiral 
groups. —  A  num- 
ber of  systems  of 
grouping  have  been 
devised.  The  limits 
of  these  groups 
have  been  deter- 
mined by  the 
method  of  analysis 
used  by  the  investigator  and  by  his  judgment  of  the 
relative  agricultural  importance  of  each  group.  A 
further  element  which  limits  the  number  of  groups  is 
the  practicability  of  recognizing  distinctions  in  the  field 
based  upon  them.  The  following  table,  from  Bulletin  24 
of  the  United  States  Bureau  of  Soils,  exhibits  the  most 
generally  known  of  these  systems  of  grouping  employed 
in  mechanical  analysis.  Some  of  these  multiply  groups 
in  the  small  particles,  while  others  give  prominence  to 
the  sand  particles. 


FIG.  18.  Silt  soil,  photomicrograph.    Magnified 
about  110  diameters.   Stained  so  that  differences 


pos 
The 


as  in  Fig.  17.  The  particles  have  the  same  char- 
acteristics as  those  of  fine  sand.  Some  of  the 
smallest  particles  are  of  the  size  of  clay. 


r^Oui^r 

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^  ^ 

TEXTURAL  GROUPS 
TABLE  VIII  a 


73 


Number  of 
group 

Hilgsrd 
m.m. 

Osborne 
m.m. 

U.  S.  Bureau 
of  Soils 
m.m. 

Hopkins 
m.m. 

1  

3.000 

3.00 

2.000 

1.0000 

2  

1.000 

1.00 

1.000 

0.3200 

3  

0.500 

0.50 

0.500 

0.1000 

4  

0.300 

0.25 

0.250 

0.0320 

5  

0.160 

0.05 

0.100 

0.0100 

6  

0.120 

0.01 

0.050 

0.0032 

7. 

0.720 

0.005 

0.0010 

8  

0.047 

9  
10  

0.036 
0.025 

11  

0.016 

12. 

0.010 

13  

Of  these  systems,  that  of  the  Bureau  of  Soils  has  been 
applied  to  the  largest  number  of  samples  and  is  most 
widely  known.  The  names  which  it  applies  to  its  dif- 
ferent groups  or  separates  are  as  follows: 

1.  Fine  gravel 2.000-1.000  m.m. 

2.  Coarse  sand 1 .000-0.500  m.m. 

3.  Medium  sand 0.500-0.250  m.m. 

4.  Fine  sand 0.250-0.100  m.m. 

5.  Very  fine  sand 0.100-0.050  m.m. 

6.  Silt 0.050-0.005  m.m. 

7.  Clay 0.005-0.000  m.m. 

All  that  material  above  two  millimeters  in  diameter 
is  classed  as  gravel  and  stone,  and  in  any  complete 
examination  must  also  be  taken  into  account.  The 
material  resulting  from  the  above  analysis  is  sometimes 
termed  the  fine  earth,  in  distinction  from  the  gravel, 
etc.  That  there  are  distinctions  which  should  be  made 
between  the  grades  of  gravel  is  obvious,  for  small 


74 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


pebbles  constitute  a  very  different  condition  from  large 
boulders  in  all  phases  of  tillage. 

The  relative  dimensions  of  the  particles  in  the  groups 
may  be  illustrated  graphically  by  the  following  diagram. 


.5-. 25      M.M.     .25-. 10      M.M.     .10-. 05  M.M.     .05-. 005  M.M.    005-.000 


FIG. 20.    Diagram  illustrating  the  relative  size  of  the  groups  of  particles,  made 
in  mechanical  analysis  by  the  Bureau  of  Soils  Classification 

26.  Agricultural  classes  based  on  texture.  —  Ob- 
viously, no  natural  soil  is  composed  entirely  of  material  like 
any  one  of  these  groups,  but  a  soil  may  contain  a  large 
proportion  of  material  of  any  one  size.  Thus,  a  sandy 
soil  is  one  containing  a  large  proportion  of  sand  par- 
ticles, and  the  coarser  the  sand  or  the  larger  its  propor- 
tion the  more  sandy  the  soil  appears.  A  clay  soil  is  one 
containing  a  large  proportion,  but  not  necessarily  a 
larger  quantity  of  clay  than  of  material  of  any  other 
size.  A  given  amount  of  fine  particles  has  a  larger 
effect  on  the  properties  of  the  soil  than  the  same  amount 
of  coarse  particles.  The  presence  of  silt  particles  in 
addition  to  clay  serves  to  make  a  soil  more  heavy  than 
if  the  same  quantity  of  sand  were  substituted  for  the  silt. 


MECHANICAL  COMPOSITION  OF  SOILS 


75 


Fio.  21.  Fine  sand  soil,  showing  the  mechanical  composition.  Each 
vial  contains  the  proportion  of  particles  of  given  size  found  in  the  samples. 
Clay  on  the  right;  fine  gravel  on  the  left.  For  key  to  sizes,  see  Fig.  19  and 
page  73. 


Fio.  22.    Silt  loam,  showing  the  mechanical  composition.    For  explanation, 
see  Fig.  21 


76 


THE   PRINCIPLES   OF  SOIL  MANAGEMENT 


A  mixture  of  all  the  groups  without  the  preponderance 
of  the  properties  of  any  one  group  constitutes  a  loam 
soil. 

For  purposes  of  a  soil  survey,  a  classification  is  made 
that  permits  of  finer  distinctions.  The  textures  which 
have  been  recognized  are  given  in  the  table  opposite, 
together  with  the  limits  in  mechanical  composition 


FIG.  23.     Heavy  clay,  showing  the  mechanical  composition.    For  explanation, 
see  Fig.  21.   Compare  with  Figs.  21,  22. 

which  they  represent.  It  is  of  course,  impossible  to  fix 
all  of  the  limits  in  such  a  classification,  and  therefore 
only  certain  groups  are  specified.  This  scheme  has 
been  devised  by  the  United  States  Bureau  of  Soils  in 
its  soil-survey  work. 

All  those  soils  having  the  same  general  texture, 
although  they  may  have  been  derived  in  a  very  different 
way,  constitute  a  soil  class.  Thus  there  is  the  sandy 
loam  class,  the  silt  class,  the  clay  class,  etc.  The  fol- 
lowing curves  exhibit  the  average  composition  of  several 


AGRICULTURAL  CLASSES   OF  SOIL 
TABLE    IX 


77 


l 

Fine 
Gravel 

2.-1. 
m.m. 

2 

Coarse 
Sand 

1.-.5 

in  in. 

3 
Me- 
dium 
Sand 

.5-  .25 

ii  i.i  n. 

4 

Fine 
Sand 

.25  -.10 

m  .m  . 

5 
Very 
Fine 
Sand 

.10  -.05 

in.  in. 

6 

Silt 

.05-  .005 
m.in. 

7 

Clay 

.005-0 
m.m. 

Coarse 
sand 

More  than  25 

%(l+2) 

0-15 

0-10 

More  than  50% 

(1+2  +  3) 

Less  than  20% 

(6  +  7) 

Medium 
sand 

Less   than  20% 
(1+2) 

0-15 

0-10 

More  than  20% 
(1+2  +  3) 

Less  than  20% 

(6  +  7) 

Fine 
sand 

Less  than  20% 

(1+2  +  3) 

0-15 

0-10 

Less   than   20%    (6  +  7) 

Sandy 
loam 

More  than  20% 

(1+2  +  3) 

10-35 

5-15 

More  than  20%  and  less 
than  50%  (6  +  7) 

Fine 
sandy 
loam 

Less 

( 

than  5 

L+2  +  l 

»% 

0 

10-35 

5-15 

More  than  20%  and  less 
than  50%  (6  +  7) 

Loam 

15-25 

Less  than 

55%  (6) 

More  than  50%   (6  +  7) 

Silt  loam 

More  than 
55%  (6) 

Less  than 
25%  (7) 

Clay  loam 

25-55               25-35 

More  than  60%  (6  +  7) 

Sandy  clay 

Less  than        More  than 
25%  (0)          20%  (7) 

Less  than  60%  (6  +  7) 

Silt  clay 

I 

More  than 
55%  (6) 

25%-35% 

(7) 

Clay 

More  than 
35%  (7) 

More  than  60%  (6  +  7) 

78 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


classes,  as  they  are  found  in  the  field.  The  field  classi- 
fication may  not  be  strictly  in  accord  with  the  mechani- 
cal analysis,  for  the  reason  that  the  same  essential 
conditions  may  result  from  more  than  one  mixture  of 
groups.  By  experience  much  facility  in  judgment  may 
be  attained. 


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SOIL  SEPARATES 

Fio.  24.    Curves  representing  the  average  analysis  of  seven  common 
field  classes  of  soil 

Taking  the  soils  formed  in  the  same  general  way, 
alluvial  for  example,  they  are  found,  to  exhibit  all 
gradations  of  fineness  from  clay  up  to  the  coarsest 
gravel  and  stony  loams.  All  these  classes  constitute 
a  soil  series.  In  the  same  way,  there  may  be  a  glacial 
series  or  even  several  of  them,  lacustrum  series,  residual 
series,  etc.  The  river-bottom  soils  of  the  Central  states 
are  chiefly  classified  by  the  Bureau  of  Soils  into  the 
Wabash  and  Waverly  series.  Some  of  the  glacial  soils 
into  Miami,  Volusia,  etc.;  coastal  plain  soils  into  Norfolk 
(yellow),  Orangeburg  (red),  etc.,  through  all  the  divis- 
ions, provinces  and  groups. 


TEXTURE   AND   CROP   RELATIONS 


79 


This  means  that,  while  sandy  loams  or  silt  loams  as 
a  class  are  similar  in  texture,  they  may  differ  in  many 
other  properties  of  importance  in  plant  production.  A 
complete  series  is  one  in  which  all  the  possible  classes 
are  represented. 

Some  idea  of  the  relation  of  these  classes  of  soil  to 
crops  is  given  by  the  following  curves.  These  soils  are 
especially  suited  to  the  production  of  the  crops  with 
which  they  are  associated. 


-1-  NORFOLK,    COARSE  SAN 


FOLK 


.?--  NORFOLK  SAND 


_X3_    COLORADO  FINE   SANDY 


_« WABA8K  LOAM 


_"*,_  MIAMI  C 


SOIL  SEPARATES 

Fio.  25.   Curves  showing  the  relation  of  soil  texture  to  crop  adaptation 

27.  Some  physical  properties  of  arid  and  humid 
soils.  —  In  discussing  the  formation  of  soils,  attention 
was  directed  to  the  effect  of  climate  upon  the  process, 
and  it  was  noted  that  under  arid  conditions  physical 
disintegration  is  likely  to  predominate  over  chemical 
decomposition,  which  results  in  an  average  coarser  text- 
ure of  the  soil.  This  appears  especially  in  the  greater 
proportion  of  soils  of  the  sandy  and  loam  classes  to 
those  of  the  silt  and  clay  classes. 

Climate  also  exercises  a  modifying  effect  as  between 


80  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

the  soil  and  the  subsoil.  In  humid  regions,  the  large 
rainfall  and  consequent  seepage  through  the  soil  is 
associated  with  a  greater  degree  of  fineness  in  the  sub- 
soil than  in  the  soil.  On  the  other  hand,  in  arid  regions 
where  there  is  not  this  large  rainfall,  and  consequent 
leaching,  the  subsoil  is  not  finer  than  the  soil,  and,  in 
fact,  is  inclined  to  be  more  coarse. 

28.  Some  properties  of  soil  separates  and  classes.— 
As  has  been  indicated,  the  justification  for  a  study  of 
individual  soil  particles  from  an  agricultural  standpoint 
is  in  their  fundamental  relation  to  the  management  of 
the   soil.     Every   farmer  is   well   acquainted   with   the 
striking  difference  in  crop  relations  and  tillage  properties 
of  sand  and  clay.    He  well  knows  that  they  must  be 
managed  differently  and  are  suited  to  different  crops. 
He  knows  sand  to  be  better  suited  to  early  maturing 
crops,  like  truck,  than  to  late  crops  and  the  grasses. 
He  knows  that  one  does  not  withstand  dry  weather, 
while   the    other   will    carry    a    crop    through    a    long 
period    of    drought.      The    cause    traces    back    to    the 
size  and  consequent  properties  of  the  soil  units.     This 
will    appear    more    clearly    in    the    discussion    of    soil 
moisture. 

29.  Number   of   particles. — Since   soil    particles  run 
to  very  small  diameters,  the  number  in  any  given  mass 
or  volume  is  very  great.    This  is  shown  in  the  following 
table,  which  gives  the  number  of  particles  in  1  gram 
(1  Ib.  equals  453.6  gr.)  of  each  of  the  fine  earth  separates, 
considering  the  particles  to  be  spheres  of  mean  diameter 
and  of  specific  gravity  2.65. 

If  the  particles  of  a  soil  are  assumed  to  be  spheres 


NUMBER   OF   SOIL   PARTICLES  81 

of  uniform  diameter  and  weight,  the  number  in  a  given 
mass  of  soil  may  be  calculated  from  the  following 
formula: 

W  W 


N 


rD3x2.65 


6 

Where  N  =  Number  of  particles. 
W  =  Weight  of  soil  used. 
R  =  Mean  radius  in  centimeters. 
D  =  Mean  diameter  in  centimeters. 
f  irR3  =  Volume  of  sphere. 

For  example,  the  mean  diameter  of  the  medium  sand 
class  is  .0875  centimeters,  and  in  3.5  grams  of  this 
material  there  would  be 

N  = 


From  the  mechanical  analysis  which  gives  the  weight 
of  each  class  of  particles  in  a  given  amount  of  soil,  the 
number  of  particles  of  each  size  may  be  calculated 
by  use  of  the  above  formula,  and  the  sum  of  the  particles 
in  each  class  gives  the  total  number  in  the  sample. 

TABLE  X.  —  NUMBER  OF  PARTICLES  IN  ONE  GRAM  OF  PURE  SOIL 
SEPARATE,  SUPPOSING  THAT  ALL  PARTICLES  ARE  SPHERICAL 

Diameter  in          Number  of  particles 
m.m.  in  one  gram 

Fine  gravel    ..............  2.000-1  .000  252 

Coarse  sand  ..............  1.000-0.500  1,723 

Medium  sand  .............  0.500-0.250  13,500 

Fine  sand  ................  0.250-0.100  132,600 

Very  fine  sand  ............  0.100-0.050  1,687,000 

Silt  ......................  0.050-0.005  65,100,000 

Clay    ....................  0.005-0.000       45,500,000,000 


82  THE   PRINCIPLES  OF  SOIL  MANAGEMENT 

Since  normal  field  soils  are  mixtures  in  different 
proportions  of  these  groups,  the  number  of  particles 
in  unit  weight  of  any  class  will  be  different  from  those 
shown  above,  and  will  not  reach  the  extreme  upper 
limits. 

The  number  of  particles  in  one  gram  of  the  classes 
of  soil  whose  analyses  are  shown  by  the  curves  on  page 
78  is  approximately  as  follows: 

TABLE  XI. — APPROXIMATE  NUMBER  OF  PARTICLES  IN  ONE 
GRAM  OF  SOIL 

Coarse  sand    3,276,000,000 

Medium  sand 3,956,000,000 

Sandy  loam 6,485,000,000 

Fine  sandy  loam ' 4,902,000,000 

Silt  loam 9,639,000,000 

Clay  loam 16,371,000,000 

Clay 19,525,000,000 

30.  Surface  area  of  soil  particles. — The  significance  of 
these  large  numbers  of  soil  particles  in  any  mass  of  soil 
lies  in  their  relation  to  the  surface  area  of  the  particles. 
These  surfaces  of  the  particles  hold  on  to  the  moisture 
the  more,  the  greater  their  area.  This  large  surface 
also  increases  the  rate  of  chemical  solution,  by  which 
the  food  constitutes  contained  in  the  mineral  particles 
become  available  for  the  plant's  use.  Another  important 
property  of  this  immense  surface  area  of  soils  is  to  retain 
food  materials  in  a  semi-available  form,  as  will  be  ex- 
plained in  discussing  the  absorptive  power  of  soils. 
(See  page  299.) 

The  surface  area  of  a  fine-textured  soil  is  greater  than 
the  first  thought  might  indicate.  This  immense  area 
exposed  by  soils  is  shown  by  the  following  table,  which 


SURFACE   AREA    OF   SOIL   PARTICLES 


83 


gives:  (1)  The  area  in  square  feet  of  one  gram  of  the 
soils  represented  by  the  curves  on  page  78.  (2)  The 
surface  area  per  pound  of  the  same  soil.  (3)  The  ap- 
proximate weight  per  cubic  foot  of  the  material  in  the 
field.  (4)  The  approximate  area  of  surface  in  one  cubit- 
foot  of  these  soils  as  they  occur  in  the  field. 

The  surface  area  of  the  particles  in  a  given  weight 
of  soil  may  be  calculated  from  the  formula. 

S  =  rD2N. 

Where  S  =  Surface  area  in  square  centimeters. 
D  =  Mean  diameter  in  centimeters. 
N  =  Number  of  particles  in  the  class  or  separate. 

Thus  in  the  calculation  on  page  81  there  were  found 
to  be  approximately  47,500  particles  in  3.5  grams  of 
medium  sand.  Their  surface  area,  provided  the  particles 
were  spherical,  would  be: 

S  =  T.0375JX  47,500  =  212  sq.  cm.  =32.8  sq.  in. 

TABLE  XII. — INTERNAL  SURFACE  AREA  OF  FIELD  SOILS  IN  SQUARE 
FEET  (Analysis  of  first  seven  represented  by  curves  on  page  78) 


• 

I 

Area  per  gram. 
Sq.  ft. 

II 

Area  per 
pound. 
Sq.  ft. 

III 
Approximate 
weight  per 
cubic  f(X)t  . 
Pounds 

IV 

Surface  area 
per   cubic   foot 
Sq.  ft. 

1.  Coarse  sand  
2.  Medium  sand  .  .  . 
3.  Sandy  loam  
4.  Fine  sandy  loam  . 
5.  Silt  loam  .  .  . 

0.8900 
1.0440 
1.8000 
1.6600 
2.9600 

405.0 
473.0 
816.0 
756.0 
1  340  0 

100 
96 
83 

82 

77 

40,500 
44,500 
66,600 
62,000 
104,000 

G.  Clay  loam  

4.0250 

1,825.0 

75 

136,500 

7.  Clay  

4.4130 

2,000.0 

71 

142,000 

8.  Sand  hill  

0.0708 

32.2 

110 

3,540 

9.  Hobart  clay  .... 

7.2820 

3,316.0 

60 

200,000 

84  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

From  this  table  it  appears  that  one  pound  of  the 
average  agricultural  soil  may  have  from  about  400 
square  feet,  in  the  case  of  coarse  sand,  to  2,000  square 
feet  internal  surface  area,  in  the  case  of  the  average 
clay.  A  more  reasonable  basis  of  comparison,  because 
of  differences  in  volume  weight,  is  that  of  one  cubic 
foot  of  the  material,  as  shown  by  the  fourth  column, 
from  which  it  appears  that  these  soils  have  from  one 
to  three  acres  of  surface  area.  These  are  striking  dif- 
ferences, particularly  those  between  soils  8  and  9, 
which  represent  extremes  in  light  and  heavy  soils, 
respectively.  Number  eight  is  the  sandrhill  soil  of  the 
Carolinas,  and  is  of  exceedingly  low  agricultural  value. 
Number  nine,  Hobart  clay,  occurs  in  eastern  North 
Dakota,  and  is  derived  from  shale  rock.  The  range  in 
surface  area  per  cubic  foot  of  these  soils  is  from  one- 
twelfth  of  an  acre,  for  the  sand,  to  almost  five  acres  for 
the  clay.  The  latter  contains  76  per  cent  of  clay  in  the 
subsoil,  the  former  2  per  cent. 

31.  Chemical  composition  of  the  soil  separates.— 
There  is  some  relation  between  the  soil  classes  or 
separates  and  their  chemical  composition.  Quartz,  Tor 
example,  in  the  original  rock  resists  decay  and  comes 
through  largely  as  sand  particles,  while  the  silicate  min- 
erals undergo  much  more  decay  which  results  in  a  larger 
proportion  of  clay  particles,  and  this  partial  difference 
in  derivation  is  reflected  in  the  composition  of  the  sepa- 
rates. The  distribution  of  plant-food  constituents  and 
the  general  chemical  composition  of  the  classes  of  a  soil 
is  shown  by  the  following  table  of  results  of  acid  anal- 
ysis, obtained  by  Loughridge  as  reported  by  Merrill. 


COMPOSITION  AND  SOLUBILITY  OF  SOIL  CLASSES    85 


TABLE  XIII 


Clay 

Fine  silt 

Coarsest 

silt 

Per  cent  present  in  soil  .... 

21.64 

23.56 

12.54 

13.67 

13.11 

Diameter  of  particles    .... 

.Oll-.OOO 
m.rn. 

.005-  .011 
m.m. 

.013-  .016 
m.m. 

.022-  .027 
m.m. 

.033-  .038 

m.m. 

Constituents 

Per  cent 

Per  cent 

Per  cent 

Per  cent 

Per  cent 

1.  Insoluble  residue... 

15.96 

73.17 

87.96 

94.13 

96.52 

2.  Soluble  silica  (Si()2) 

33.10 

9.95 

4.27 

2.35 

3.  Aluminum  (A12O3). 

18.19 

4.32 

2.64 

1.21 

4.  Ferric  iron  (Fe^Oj)  . 

18.76 

4.76 

2.34 

1.03 

5.  Phosphoric  anhy- 

drid(PA)    ...- 

0.18 

0.11 

0.03 

0.02 

6.  Sulfur        trioxid 

(SO3) 

006 

002 

003 

003 

7.  Lime  (CaO)    . 

009 

0  13 

0  18 

009 

8.  Magnesia  (MgO)  .  . 

1.33 

0.46 

0.26 

0.10 

9.  Soda  (Na3O)  

0.24 

0.28 

0.21 

10.  Potash  (K,O)  .... 

1.47 

0.53 

0.29 

0.12 

11.  Volatile  matter  .  .  . 

9.00 

5.61 

1.72 

0.92 

Totals  

99.84 

99.30 

100.00 

100.21 

Total  soluble  constitu- 

ents   

75.18 

20.52 

10.32 

5.16 

This  table  illustrates,  (1)  The  much  greater  solu- 
bility of  the  fine  particles  in  strong  hydrochloric  acid. 
(2)  That  the  absolute  amount  of  food  elements  dis- 
solved is  greater  in  the  fine-textured  class  than  in  the 
coarse-textured  class.  (3)  That  the  ratio  of  food  ele- 
ments dissolved  to  the  aluminum  and  other  refractory 
constituents  dissolved  is  narrower  in  the  coarse  than  in 
the  fine-textured  class. 

Failyer's  results  are  summarized  in  the  following 
table. 


3 

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(86) 


MODIFICATION   OF  SOIL   TEXTURE  87 

These  figures,  and  those  published  by  a  number  of 
other  experimenters,  clearly  show  the  larger  portion  of  the 
phosphorus,  calcium,  magnesium  and  potassium  in  the 
fine-textured  classes  in  all  kinds  of  soil.  The  absolute 
amount  of  the  food  elements  is  also  greatest  in  the  fine 
separates.  It  is  shown  that  those  soils  which  have 
undergone  the  greatest  weathering — the  coastal  plain 
soils — are  much  the  lowest  in  the  food  elements  through- 
out the  different  classes.  On  the  other  hand,  glacial 
soils  are  relatively  rich  in  these  food  elements.  There 
is  also  much  less  difference  in  composition  between 
the  clay  and  the  sand  particles  in  glacial  soils,  presumably 
because  these  soils  have  been  formed  largely  by  mechani- 
cal processes,  without  much  weathering  or  leaching. 
The  arid  soils  presented  are  not  fully  representative, 
but  they  illustrate  the  high  percentages  of  the  food 
elements  in  all  the  classes  of  particles,  although  the 
same  concentration  in  the  fine  particles  is  apparent. 

It  is,  therefore,  concluded  that  clay  particles  are 
relatively  richer  in  food  elements  than  sand  particles. 
But  in  glacial  and  arid  soils,  and  to  a  degree  in  residual 
soils,  the  sand  particles  are  much  richer  in  food  elements 
than  they  are  in  soils  of  water-deposition,  such  as  the 
coastal  plain. 

32.  Modification  of  soil  texture. — The  only  feasible 
method  of  changing  the  texture  of  a  soil  is  by  adding 
to  it  material  of  a  different  texture.  Thus,  the  green- 
house man  considers  the  requirements  of  his  crops, 
and  by  mixture  of  fine  and  coarse  material  obtains  the 
texture  which  is  necessary  for  their  best  development. 
This  is  entirely  practicable  where  only  a  small  volume 


88  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

of  soil  is  involved,  but  under  field  conditions  modifica- 
tions of  texture  artificially  are  not  practicable,  because 
of  the  expense  involved.  The  farmer  must  generally 
accept  the  texture  of  the  soil  as  he  finds  it,  and  make 
the  best  of  his  conditions  by  suitable  selection  of  crops 
adapted  to  his  soil,  and  by  such  modifications  of  the 
structure  of  the  soil  as  its  texture  will  permit. 

33.  Structure. — Soil  structure  deals  with  the  arrange- 
ment of  the  soil  particles  independently  of  their  size. 

34.  Some  aspects  of  soil  structure. — The  arrangement 
of  the  soil  particles  may  be  viewed  in  many  different 
ways.     Upon    this    arrangement    depend    several    very 
important   physical   properties,   which  in  turn  have  a 
fundamental  bearing  on  chemical  and  biological  prop- 
erties. 

35.  Ideal  arrangements. — Taking   the  simplest  case 
first,  that  of  spherical  particles  of  one  size,  these  may 
be  arranged  in  general  forms:    (1)  In  columnar  order, 
with  each  particle  touching  its  neighbors  at  only  four 
points.    (2)  In  oblique  order,  with  each  particle  touch- 
ing its  neighbors  at  six  points.    (3)  These  spheres  may 
be   gathered   into   larger   spheres   which   rest   together 
in  the  second  order.    In  the  first  the  unoccupied  or  pore 
space  is  47.64  per  cent  of  the  total  volume  occupied 
by  the  spheres.    In  the  second  it  is  25.95  per  cent.    In 
the  third  case,  however,  where  there  are  spheres  within 
spheres,  the  pore  space  is  greatly  increased — to  74.05 
per  cent.    (4)  On  the  other  hand,  if  there  are  spheres  of 
several  sizes  so  that  the  small  ones  may  rest  in  the 
spaces  between  the  large  ones,  the  total  pore  space  will 
be  reduced  below  25.95  per  cent,  and  the  spaces  may 


ARRANGEMENT  OF  SOIL  PARTICLES 


89 


continue  to  be  filled  in  by  smaller  spheres  until  the 
mass  is  practically  solid,  without  pores.  (See  Fig.  26.) 
It  is  of  course  recognized  that  under  field  conditions 
these  ideal  arrangements  do  not  pertain,  but  these 
figures  illustrate  the  underlying  factors  which  determine 
differences  in  pore  space,  and,  also,  differences  in  other 
physical  properties.  Soil  particles  are  irregular  in  shape 
and  uneven  in  size.  When  brought  very  close  together, 


FIG.  26.  Ideal  arrangements  of  spherical  soil  particles:  (1)  Columnar  order, 
47.64  per  cent  of  pore  space.  (2)  Oblique  order,  25.95  per  cent  of  pore  space. 
(3)  Compound  spheres  in  oblique  order,  74.05  per  cent  of  pore  space.  (4)  Three 
sizes  of  spheres  with  closest  packing,  about  5  per  cent  of  pore  space. 

as  occurs  in  mixing  in  a  wet  condition,  their  molecular 
attraction  is  brought  into  operation  and,  especially 
when  dry,  they  are  held  together  very  securely.  In  this 
way  the  normal  molecular  attraction  of  the  soil  particles 
is  increased  by  the  deposition  around  them  of  the 
material  in  solution. 

Applying  these  principles  to  the  soil,  it  is  observed 
that  there  may  be  two  general  arrangements  of  the 
particles.  (1)  Each  particle  may  be  separate  and  free 
from  its  neighbors.  This  is  a  separate-grain  structure. 
That  is,  each  particle  of  soil  functions  separately.  When 
by  proper  manipulation  the  particles  are  so  packed 
together  that  the  small  particles  quite  completely  fill  in 
the  spaces  between  the  large  ones,  so  that  a  very  dense 


90 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


mass  is  formed  (Fig.  26,  No.  4),  the  structure  is  termed 
puddled.  The  term  puddled,  in  this  connection,  is  re- 
lated to  the  fact  that  such  an  arrangement  can  be 


Fto.  27.  An  example  of  undesirable  structure.  A  clay  soil  which  had  been 
puddled  by  tramping  when  wet.  "Bad  tilth. "Compare  with  Fig.  28,  showing 
'ideal  tilth."  Note  also  a  type  of  auger  used  in  examining  soils.  A  common 
one  and  one-half  inch  wood  auger  welded  to  a  one-half-inch  shank,  giving  a 
total  length  of  about  three  feet. 

obtained  only  in  fine-textured  soils  when  they  are  mixed 
(puddled)  in  a  very  wet  condition,  so  that  the  fine 
particles  will  move  into  the  large  spaces. 


GOOD   AND   BAD    TILTH 


91 


On  the  other  hand,  the  small  particles  may  adhere 
to  the  large  ones,  or  a  number  of  small  particles  may 
adhere  together  as  a  group  or  granule.  When  a  number 
of  united  particles  function  together  as  a  single  larger 
particle  or  granule,  the  structure  is  termed  granular. 


FIG.  28.     Ideal  tilth  of  a  soil 

This  arrangement  is  also  termed  the  crumb  structure. 
According  as  these  groups  are  prominent  or  incon- 
spicuous, the  soil  is  said  to  be  well  or  poorly  granulated. 
But  when  the  granules  reach  large  size,  so  that  they 
interfere  with  the  best  functioning  of  the  soil,  they  are 
termed  clods.  That  is,  a  clod  is  an  unsizable  granule. 


92  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

It  is  well  known  that  a  box  of  baseballs,  or  a  pile 
of  boulders,  or  even  a  box  of  sand,  does  not  adhere 
together  to  any  appreciable  extent.  That  is,  in  all  the 
coarser -textured  classes,  certainly  down  to  the  size 
of  very  fine  sand,  there  is  very  little  tendency  to  granu- 
late. But  in  the  silt,  to  a  small  extent,  and  in  the  clay, 
to  a  very  great  extent,  granulation  is  strong. 

36.  Porosity. — In  a  mass  of  particles  there  is  some 
unoccupied  or  pore  space.  If  the  particles  are  fine,  then 
the  intervening  spaces  are  correspondingly  small;  if 
large,  the  spaces  are  large.  In  the  discussion  of  ideal 
particles  above,  it  was  shown  that  the  pore  space  is 
theoretically  independent  of  the  size  of  the  particles, 
with  any  given  arrangement.  There  would  be  as  much 
pore  space  in  a  cubic  foot  of  buckshot  as  in  one  of 
marbles.  But  in  the  soil  this  is  not  true.  For,  the  finer 
the  particles,  the  larger  the  proportion  of  pore  space 
is  found  to  be. 

A  clay  has  much  more  total  pore  space  than  a  sand, 
although  the  individual  spaces  or  openings  between 
the  particles  are  much  smaller  in  the  clay.  The  approxi- 
mate per  cent  of  pore  space  in  a  soil  may  be  calculated 
by  use  of  the  following  formula. 

Vw 
Vs- 
p_       2.66  _Vp 

\s  Vs 

Where  P  =  Per  cent  of  pore  space. 

Vs=- Volume  in  c.c.  occupied  by  the  soil. 
Vw  =  Weight  of  water  equal  to  weight  of  soil  in  grams. 
Vp  =  Volume  in  c.c.  of  pore  space  in  soil. 
2.65  — Specific  gravity  of  soil  particles. 


POROSITY   OF  SOIL  93 

Another  and  more  simple  formula  which  may  be 
used  in  the  calculation  of  the  pore  space  is  as  follows: 

p=ioo-Ap;sp-gr-xioo 

Ab.  sp.  gr. 

Where  P  =  Per  cent  of  pore  space. 
Ap.  Sp.  =  Apparent  specific  gravity  or  volume  weight. 
Ab.  Sp.  =  Absolute  specific  gravity  of  soil  material. 
100%  =  Total  space  occupied  by  soil  mass. 

This  relation  between  texture  and  pore  space  is 
exhibited  by  the  following  table  of  figures  for  soils  in 
field  condition. 

Per  cent  by  volume 

1.  Clean  sand 33.50 

2.  Coarse  sand 40.00 

3.  Medium  sand 41.80 

4.  Fine  sand 44.10 

5.  Sandy  loam 51.00 

6.  Fine  sandy  loam 50.00 

7.  Silt  loam    53.00 

8.  Clay  loam    54.00 

9.  Clay    50.00 

10.  "Gumbo"  clay  (Wedgefield) 58.46 

11.  Heavy  clay  (Potomac  puddled) 47.19 

12.  Very  heavy  clay  (pipe  clay) 65.12 

The  reason  for  the  greater  porosity  of  the  finer  soils 
appears  to  be,  that  the  smallest  particles  are  so  light 
that  they  do  not  settle  so  closely  together  in  proportion 
to  their  size  as  do  the  sand  particles,  because  of  the 
greater  friction  between  their  surfaces.  When  this  is 
overcome  by  mixing  in  water,  such  material  becomes 
dense.  Treatment  greatly  affects  the  structure  and 
therefore  the  porosity  of  the  soil.  This  is  well  shown 


94  THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

by  figures  from  the  Rothamsted  fields.  The  porosity 
of  the  surface  nine  inches  of  soil  in  an  old  pasture  was 
56.8  per  cent,  while  in  the  same  depth  of  a  cultivated 
field  it  was  45.5  per  cent.  Extensive  areas  of  loam  soils 
in  the  North  Central  states  have  a  porosity  of  from 
45  per  cent  to  49.6  per  cent.  In  many  of  the  heavier 
soils  it  much  exceeds  50  per  cent,  and  in  well-granu- 
lated clays  it  may  reach  70  per  cent,  or  in  light  sand 
it  may  be  less  than  40  per  cent.  In  general,  it  may  be 
said  that  about  one-half  of  the  volume  of  ordinary 
cultivated  soils  of  intermediate  texture  is  pore  space. 

The  diameter  of  the  individual  pore  spaces  is  of 
importance,  as  well  as  the  total  volume  of  pore  space, 
since  these  determine  the  capacity  of  the  soil  to  retain 
and  move  water  and  to  permit  the  circulation  of  gases 
in  the  soil  mass,  as  well  as  to  facilitate  the  extension  of 
the  plant -roots. 

37.  Weight. — The  weight  of  soil  is  the  result  of  two 
factors.  These  are,  first,  the  absolute  weight  of  the  indi- 
vidual particles,  or  absolute  specific  gravity,  and  second, 
the  volume  of  pore  space  in  the  mass. 

By  reference  to  the  table  of  minerals  on  page  6, 
it  will  be  seen  that  the  minerals  entering  into  the  soil 
vary  greatly  in  specific  gravity — that  is,  their  weight 
as  compared  with  an  equal  volume  of  water.  They 
range  from  about  2.5  to  6  or  8,  but  the  minerals  which 
make  up  the  great  bulk  of  the  soil — quartz,  feldspars, 
micas,  calcite,  etc., — all  have  a  specific  gravity  of  from 
2.6  to  2.8.  (See  Table  I,  pages  6  and  7.)  Many  deter- 
minations of  this  property  have  been  made.  Fineness 
does  not  appear  to  have  any  material  effect  upon  it. 


WEIGHT   OF   SOIL 


95 


Whitney  has  obtained  the  following  specific  gravities  of 
composite  soil  separates. 

TABLE  XV 


Conventional  name 

Diameter  (m.m.) 

Specific  gravity 

Fine  gravel  

2-1 

2.647 

Coarse  sand  

1-.5 

2.655 

Medium  sand       .        .        .    . 

.S-.25 

2.648 

Fine  sand 

.25-.10 

2.659 

Very  fine  sand  

.1-.05 

2.680 

Silt  

.050-.005 

2.698 

Clay.  . 

.005-.000 

2.8.37 

There  is  a  very  small  increase  in  the  specific  gravity 
of  the  clay  group,  probably  due  to  the  greater  concentra- 
tion of  the  iron  compounds  here  as  a  result  of  chemical 
processes:  but  it  is  not  sufficient  to  materially  change 
the  result.  The  average  specific  gravity  of  soil  material 
is,  therefore,  usually  taken  as  2.65,  and  this  figure  is 
used  in  all  calculations  here  given. 

Since  the  pore  space  enters  into  the  calculation  of 
the  weight  of  any  volume  of  field  soil,  this  figure  is 
much  more  variable  for  different  soils  than  the  one  just 
given.  It  is  directly  related  to  pore  space,  and  the 
larger  the  volume  of  pore  space,  the  smaller  the  unit 
weight. 

Combining  the  figures  for  pore  space  given  above 
with  that  for  average  specific  gravity,  the  figures  in 
the  following  table  are  obtained. 

The  weight  of  a  given  volume  of  soil  may  be  deter- 
mined from  the  pore  space  and  specific  gravity  of  the 
material,  by  use  of  the  following  formula. 


96 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


Ws  =  WwX  (2.65  X  (100 -P). 
Where  Ws  =  Weight  of  given  volume  of  soil. 

Ww  =  Weight  of  volume  of  water  equal  to  volume  of  soil. 

P  =  Per  cent  of  pore  space. 
(100  — P)  =  Per  cent  of  volume  occupied  by  soil. 

Or  the  following  formula  may  be  used,  and  is  often 
more    convenient. 

Ws=Ap.  Sp.xWw. 
Where  Ws  =  Weight  of  soil. 

Ap.  Sp.  =  Apparent  specific  gravity. 

Ww  =  Weight  of  volume  of  water  equal  to  that  occupied 
by  the  soil. 

TABLE  XVI 


Volume  weight 
or  apparent 
specific  gravity 

Weight  per 
cubic  foot 

Weight  per 
acre  foot 

Kilo- 
grams 

Lbs. 

Lbs. 

1  .  Clean  sand  

1.76 
1.60 
1.54 
1.48 
1.30 
1.32 
1.24 
1.22 
1.17 
1.10 

1.39 
0.93 

1.14 

1.43 
1.44 
1.33 

50.0 
45.5 
43.5 
42.0 
36.8 
37.4 
35.2 
34.5 
33.1 
31.2 

39.6 
26.3 

32.3 

40.5 
41.0 

37.8 

110.0 
100.0 
96.0 
93.0 
81.0 
82.5 
77.5 
76.0 
72.6 
68.5 

87.2 
58.0 

71.0 

89.0 
90.0 
83.0 

4,800,000 
4,356,000 
4,200,000 
4,080,000 
3,550,000 
3,590,000 
3,400,000 
3,330,000 
3,180,000 
3,000,000 

3,820,000 
2,540,000 

3,100,000 

3,900,000 
3,940,000 
3,640,000 

2    Coarse  sand         

3.  Medium  sand         .  .            ... 

4.  Fine  sand.        .        .            ... 

5.  Sandy  loam             .            .  . 

6    Fine  sandy  loam.    . 

7.  Silt  loam  

8.  Clay  loam  

9.  Clay  

10.  "Gumbo"  clay  

11.  Puddled  heavy  clay    (Poto- 
mac)     

12.  Heavy  pipe  clay    

13.  Old  pasture  clay  loam  (Roth- 
amsted)  

14.  Cultivated   soil,    clay   loam 
(Rothamsted)  

15.  Hagerstown  loam   

16.  Janesville  loam       

One  kilogram  =»2.2  pounds. 


PLASTICITY    OF   SOIL  97 

This  table  shows  that  the  finer  the  soil  the  lighter 
its  absolute  weight.  Clay  soils  may  range  from  60  to  90 
pounds  in  weight,  according  to  their  fineness  and  state 
of  granulation.  Sand  soils  weigh  from  90  to  110  pounds. 
In  practice,  soils  are  spoken  of  as  "light"  and  "heavy,"  ^y^ 
but  this  use  of  these  terms  does  not  apply  to  the  weight 
of  the  soil.  The  term  light  is  applied  to  sandy  soil 
because  the  particles  move  freely.  On  the  other  hand, 
a  clay  is  termed  heavy  because  of  its  cohesiveness. 

38.  Plasticity. — The  property  of  stickiness  of  soils, 
when  mixed  with  water,  is  termed  plasticity.  Soils 
exhibit  it  in  very  different  degrees.  In  general,  it  may 
be  said  that  the  finer  the  soil  the  greater  the  plasticity, 
and  therefore  the  finest-textured  clays  generally  exhibit 
the  greatest  degree  of  plasticity.  On  the  other  hand, 
plasticity  is  not  absolutely  lacking  in  sandy  soil,  for, 
when  moist,  this  material  adheres  together  and  may 
support  a  considerable  weight.  But,  when  the  water  is 
removed  by  drying,  the  sand  will  fall  apart  readily, 
and  therefore  the  cohesiveness  exhibited  was  largely 
due  to  the  surface  tension  of  the  water  between  the 
particles.  However,  when  the  clay  is  dried  out.  it  be- 
comes a  hard  mass,  and  it  has  a  superior  adhesive  and 
cohesive  property  when  dried  from  the  wet  puddled 
state. 

But  while  plasticity  and  great  tensile  strength  appear 
to  be  very  closely  associated  with  fine  texture,  the 
fineness  does  not  appear  to  be  entirely  responsible  for 
the  property,  as  is  shown  by  the  results  of  numerous 
studies  reviewed  by  Ries.  Writers  in  the  past  have 
dwelt  much  on  the  effect  of  colloidal  clav  in  this  connec- 


98  THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

tion.  The  real  significance  of  colloidal  material  is  some- 
what doubtful,  and,  further,  the  amount  present  in 
even  the  most  plastic  clays  is  so  small  as  hardly  to  be 
given  credit  for  the  effects  noted.  It  seems  probable 
that  plasticity  and  cohesiveness  of  the  material  is  due 
to  several  uniting  causes,  but  for  all  practical  purposes 
of  the  farmer  it  may  be  identified  with  fineness  of  tex- 
ture. Associated  with  plasticity  is  a  certain  amount  of 
shrinkage  upon  drying,  and  expansion  upon  wetting. 
The  checking  of  the  clay  soil  is  an  example  of  this. 
As  the  water  dries  out  of  the  soil,  the  surface  film  draws 
continually  closer  about  the  particles,  and,  if  these 
are  small  enough,  may  move  them  closer  together. 
Then,  if  the  whole  mass  is  not  drawn  together  as  one 
unit,  there  will  be  cracks  developed  as  a  result  of  the 
shrinkage.  The  cracks  occur  where  there  is  a  weakness, 
from  whatever  cause,  in  the  structure  of  the  soil.  War- 
ington  reports  the  results  of  Schiibler,  which  show  that 
a  very  pure  clay,  when  dried  from  a  thoroughly  puddled 
condition,  contracted  18.3  per  cent  of  its  original  vol- 
ume; a  sandy  clay  contracted  6  per  cent,  and  a  sample 
of  humus,  20  per  cent  of  its  volume.  Gallagher  found 
a  shrinkage  of  over  30  per  cent  in  drying  out  a  sample 
of  muck.  These  figures  illustrate  the  general  fact  that 
the  finer  the  texture  the  greater  the  shrinkage.  Con- 
versely, on  wetting,  there  is  a  similar  though  smaller 
degree  of  expansion. 

The  checking  of  soil  resulting  from  this  shrinkage 
may  be  very  injurious  to  crops.  Where  large  checks 
or  cracks  are  formed,  the  roots  of  plants  may  be  injured 
or  broken.  And,  further,  these  cracks  greatly  hasten 


CEMENTING   MATERIALS   IN  SOIL 


99 


Fio.  29.     Exci 


[•becking  of  a  heavy  clay  soil  as  a  result  of  drying.    Illus- 
trates the  process  of  soil  granulation. 


the  drying  out  of  clay  soil  to  a  much  greater  depth  than 
is  possible  through  surface  evaporation.  They  also 
interfere  greatly  with  the  advance  of  roots. 

39.  Cementing    material. — The    cohesion    of    a    soil 


100         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

when  dry  is  due  to  several  causes,  one  of  which  is  much 
the  most  prominent.  This  is  cementing  materials.  A 
cementing  material  is  any  material  which  binds  sur- 
faces together.  In  a  gravel  or  sand  pit,  masses  of  the 
material  are  sometimes  found  united  into  a  conglom- 
erate rock.  After  a  protracted  dry  spell,  moist  sur- 
faces show  a  white  incrustation  in  the  surface  layer, 
which  is  due  to  the  deposition  of  the  salts  in  solution 
when  the  moisture  evaporated,  and  this  acts  as  a  bind- 
ing material.  This  is  one  of  the  main  reasons  why  a 
fully  dried  soil  is  usually  so  much  harder  than  one 
slightly  moist.  The  salts  of  many  kinds  which  were  in 
solution  in  the  moisture  have  been  deposited. 

This  composite  of  dissolved  salts  is  the  first  of  four 
common  cementing  materials  which  occur  in  the  soil. 
It  is  generally  a  weak  binding  material.  The  second 
material  is  lime.  Some  soils  are  very  rich  in  this  com- 
pound. Particularly  is  this  true  of  most  glacial  soils, 
and  in  North  Dakota  and  other  sections  of  the  country 
extended  areas  of  gravel  beds  occur,  in  which  the  upper 
two  or  three  feet  are  completely  bound  together  by  the 
deposition  of  lime  between  and  around  the  particles. 
It  has  been  leached  out  of  the  soil  above  as  bicarbonate, 
under  the  influence  of  carbonated  water  formed  by  the 
decaying  organic  matter,  but  here,  in  the  loose  gravel, 
by  the  escape  of  some  of  the  carbon  dioxid  it  was 
deposited.  This  is  the  usual  history  of  the  process. 
Cementation  by  lime  carbonate  is  a  very  common  and 
very  general  process.  The  third  cementing  material 
is  the  various  forms  of  iron — usually  oxides — in  various 
stages  of  hydration.  They  have  come  into  solution  by 


COLOR   OF  SOILS  101 

the  assistance  of  various  organic  acids,  and  are  again 
deposited  where  there  is  some  change  in  physical  con- 
ditions. This  form  is  most  common  in  the  unglaciated 
section  of  the  country  in  the  older  deposits.  Some  of  the 
red  soils  of  the  coastal  plain  region  exhibit  a  strong 
tendency  to  "case-harden," — that  is,  become  quite 
hard  at  the  surface  upon  drying,  largely  due  to  iron 
compounds.  The  fourth  cementing  material  is  silica, 
and  is  less  prominent  in  soil  practice  than  the  other 
three  cementing  materials  mentioned.  It  is  the  binding 
material  in  most  sandstones  and  quartzite  rock,  as  an 
advanced  stage  of  silica  infiltration. 

All  these  cementing  materials  except  the  iron  com- 
pounds, which  are  red,  yellow  or  brown,  are  light- 
colored. 

40.  Color. — A  great  variety  of  colors  are  exhibited 
by  soils.  These  are  not  usually  the  result  of  the  color 
of  the  individual  particles  which  make  up  the  bulk 
of  the  material.  Rather,  it  is  usually  the  result  of 
material  which  adheres  to  the  particles. 

There  are  two  chief  coloring  materials  in  soil.  These 
are  iron  compounds  and  organic  matter.  The  first  gives 
rise  to  red,  yellow,  blue  and  gray  colors.  The  latter 
gives  rise  to  some  shade  of  black  or  brown  color.  When 
these  are  combined,  various  intermediate  tints  are 
obtained.  For  example,  when  a  red  soil  is  rich  in  decayed 
organic  matter — humus — it  becomes  of  a  rich  brown 
color. 

The  color  of  soils,  especially  as  regards  iron  com- 
pounds, is  not  fully  understood,  but  it  is  safe  to  say 
that  much  color  is  the  result  of  different  forms  of  iron 


102         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

in  the  soil.  In  the  boulder  clay  of  the  glaciated  sections 
a  bluish  color  is  common,  which  seems  to  be  due  to  the 
presence  of  protoxid  of  iron  (FeO),  resulting  from 
the  great  deficiency  of  oxygen.  Where  this  comes 
in  contact  with  carbonated  water,  it  may  be  changed 
to  the  carbonate  of  iron,  which  is  gray,  and  consequently 
along  the  line  of  roots  and  in  the  bottom  of  ponds  this 
gray  color  may  be  found. 

Where  there  is  an  abundant  supply  of  oxygen,  the 
iron  takes  on  the  sesquioxid  (Fe2O3)  form,  which 
has  a  deep  red  color,  typified  by  iron  rust.  Where  the 
red  soil  stands  much  in  contact  with  water,  it  may 
become  yellow  by  the  hydration  of  the  iron  (Fe2O3  + 
H2O).  In  many  regions  a  dark-colored  soil  is  looked 
upon  as  a  fertile  soil.  This  relation  has  developed 
because  of  the  association  of  a  dark  color  with  the 
presence  of  organic  matter,  with  all  its  beneficial  effects, 
while  the  light  color  indicates  its  absence.  This  relation 
does  not  hold  universally,  but  it  is  quite  a  reliable 
guide. 

The  only  instances  where  the  color  of  the  particles 
themselves  give  color  to  the  soil  is  in  some  of  the  clean 
quartz  sands,  where  the  white  color  of  the  dominant 
mineral  gives  color  to  the  mass.  In  some  dark  shaley 
sands  this  same  principle  obtains. 

To  the  experienced  person,  the  color  of  the  soil  is  a 
valuable  guide  to  its  condition  and  productiveness. 
Mottled  and  uneven  color,  for  example,  indicates  poor 
aeration,  frequently  the  result  of  deficient  drainage. 

41.  Physical  absorption. — The  soil  particles  attract 
and  hold  materials  upon  their  surfaces.  This  physical 


MODIFICATION.  OF  SOIL  STRUCTURE  103 

absorption,  or  adsorption,  as  it  is  sometimes  called, 
is  different  from  chemical  absorption,  later  to  be  men- 
tioned, with  which  it  is  closely  associated.  As  a  result 
of  this  property,  gases  and  materials  in  solution  in  the 
soil  moisture  are  attracted  to  and,  loosely  held  by  the 
surface  of  the  soil  particles.  It  varies  with  the  extent  of 
surface  exposed,  and  is  consequently  greatest  in  fine- 
textured  soil.  In  clay  soil,  which  has  a  relatively  large 
surface  area,  it  is  very  large,  and  is  an  important  factor 
in  the  retention  of  fertilizers. 

42.  Conditions  affecting  structure. — The  arrange- 
ment of  the  particles  in  a  soil  may  be  modified  in  many 
ways.  Some  conditions  tend  to  produce  the  compact 
separate-grain  structure,  while  others  favor  the  granular 
or  crumb  structure. 

It  has  been  suggested  by  Whitney,  King  and  others, 
that  much  of  the  formation  of  granules  in  the  soil  is 
due  to  the  contraction  of  the  moisture  film  around  the 
particles,  when,  for  any  reason,  the  moisture  content 
is  reduced.  It  is  known  that  the  soil  particles  tend  to 
be  drawn  together  by  this  reduction  in  the  soil  moisture. 
Add  to  this  some  influence  to  determine  the  size  of  the 
granules  and  a  binding  material  to  permanently  hold 
the  granules  together,  and  the  essential  conditions  for 
the  granular  conditions  of  soil  are  realized.  Several 
natural  conditions,  and  the  various  tillage  operations, 
probably  exert  their  influence  on  granulation  in  this 
way.  Warington  attributes  granulation  to  unequal 
expansion  and  contraction  of  the  soil  mass,  due  to  the 
unequal  imbibition  and  loss  of  water.  In  such  a  soil, 
the  cohesive  force  being  different  in  different  parts, 


104         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

and  the  internal  strains  and  pressures  unequal,  a  ten- 
dency arises  for  the  mass  to  divide  along  the  lines  of 
weakness  into  groups  of  particles,  as  the  soil  moisture 
is  much  reduced  below  a  certain  optimum  condition. 
Tillage  operations,  development  of  roots,  burrowing  of 
animals  and  insects,  the  presence  of  humus,  and  the 
development  of  frost  crystals,  may  assist  in  further 
developing  these  lines  of  weakness  in  the  soil  mass, 
upon  which  the  tension  of  the  moisture  films  around 
the  soil  particles  is  brought  to  bear.  The  flocculation 
of  soil  particles  may  also  develop  lines  of  cleavage  by 
the  aggregation  of  particles  around  certain  centers. 
The  movement  of  the  soil  particles  is,  in  every  case, 
facilitated  by  the  presence  of  a  moderate  amount  of 
moisture. 

On  the  other  hand,  conditions  opposite  from  the 
above,  including  tillage  at  inopportune  times,  the 
operation  of  some  natural  agencies,  as  the  beating  of 
rain,  erosion,  and  bad  drainage,  may  not  only  destroy 
the  tendency  to  the  granular  condition,  which  is  always 
strongest  in  the  finest  soil,  but  may  induce  the  opposite 
or  separate  grain  structure. 

43.  Means  of  modifying  structure. — It  is  apparent 
that  some  of  the  means  of  modifying  the  soil  structure 
are  natural,  others  are  within  the  control  of  man.  The 
following  are  among  the  better-known  of  these  factors: 
(1)  Variation  in  the  water  content.  (2)  Development 
of  frost  crystals.  (3)  Tillage.  (4)  Growth  of  plant 
roots.  (5)  Organic  matter.  (6)  Certain  soluble  salts. 
(7)  Earth-worms  and  other  forms  of  animal  life.  (8) 
Heavy  rain  storms.  Whether  a  desirable  or  an  undesir- 


WATER    CONTENT    AND   SOIL   STRUCTURE 


105 


able  soil  structure  will  result  depends  upon  the  combi- 
nation of  factors  in  operation.  These  structural  modi- 
fications have  to  do  primarily  with  the  finer-textured 
soils — the  loams,  silts  and  clays, — rather  than  with 
the  sandy  soils.  The  structure  of  the  latter  can  not  be 
greatly  changed. 

44.  Variation  in  moisture  content. — The  alternate 
wetting  and  drying  of  a  clay  or  a  loam  soil  tends  to 
produce  a  granulated  structure.  It  has  been  suggested 
by  Whitney  that  this  is  due  to  the  contraction  of  the 
moisture  film  around  the 
particles,  as  it  is  reduced 
in  drying.  The  very  con- 
siderable pressure  of  the 
moisture  film  and  the  re- 
duced friction  due  to  the 
presence  of  moisture  in  the 
mass  causes  the  particles 
to  be  drawn  together  in 
small  masses.  This  process 
is  well  illustrated  by  Fig. 
30,  which  was  made  from  a 
micro-slide  in  which  was 
mounted  a  suspension  of 
fine  clay  in  water.  The 
water  slowly  evaporated 
from  under  the  cover,  and 
at  last  disappeared  along 
the  dark  lines  which  are 
formed  by  the  concen- 
tration of  the  particles  by 


FIG.  30.  Photo-micrograph,  show- 
inn  the  distribution  of  soil  particles  by 
a  water  film.  A  small  quantity  of  clay 
was  suspended  in  water  on  a  micro- 
slide  and  sealed  in  with  balsam.  Evap- 
oration wn-s  permitted  to  take  place  very 
slowly  through  n  small  opening.  The 
retreat  of  the  water  to  the  dark  border 
line  assembled  the  soil  particles  so  that 
they  were  left  to  form  the  dark  lines 
when  their  ma,ss  became  too  (treat  to  he 
moved  by  the  surface  tension  of  the 
liquid.  This  illustrates  the  granulating 
influence  of  a  contracting  water  film, 
which  is  the  primary  force  in  operation 
during  the  drying-out  of  a  wet  soil. 
Note  also  the  uniform  curvature  of  the 
film,  as  indicated  by  the  arrangement  of 
the  soil  particles. 


106         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

the  moisture  film  as  it  contracted.  The  small  particles 
are  moved  into  the  spaces  between  the  large  ones, 
thereby  reducing  the  volume,  as  is  shown  by  the  checks. 
The  checks  which  result  from  shrinkage  are  due  to  the 
unequal  contraction.  There  comes  a  time  when  the 
general  film  around  the  whole  mass  must  rupture.  It 
breaks,  along  the  line  of  least  resistance,  through  a  large 
pore  space  independently  of  how  this  space  may  have 
been  formed.  If  the  soil  mass  is  very  uniform,  there  will 
be  few  breaks,  and  the  shrinkage  will  be,  as  a  whole  or 
at  most,  around  a  relatively  few  centers.  This  process 
produces  clods  or  overgrown  granules.  But,  if  there  are 
numerous  lines  of  weakness,  there  will  be  many  centers 
of  contraction,  and  consequently  a  larger  number  of 
small  clods  or  granules  will  be  formed.  This  is  the  desir- 
able condition,  and  constitutes  good  tilth, — that  is,  the 
most  favorable  physical  condition  for  plant  growth. 

While  once  drying  produces  some  checks, — a  few 
large  ones  and  many  small  ones,  —  such  a  structure 
does  not  constitute  good  tilth.  The  process  must  be 
continued  further.  When  the  soil  is  remoistened,  it 
expands,  but  usually  not  to  its  original  wet  volume. 
Therefore  the  checks  remain  as  lines  of  weakness,  and, 
upon  a  redrying,  are  effective  in  further  reducing  the 
size  of  the  granules.  When  this  process  is  repeated 
a  number  of  times,  as  occurs  under  field  conditions, 
it  results  in  a  small  and  very  desirable  size  of  soil  granule. 
Further,  the  drying  out  of  the  water  in  the  granule 
deposits  the  salts  in  solution,  which  binds  the  particles 
together  in  a  somewhat  permanent  and  stable  aggre- 
gate. The  following  figures  represent  the  relative  force 


WETTING   AND   DRYING,    AND   STRUCTURE         107 

required  to  sink  a  knife-edge  into  a  puddled  clay  soil, 
different  samples  of  which  were  subject  to  drying  and 
rewetting  a  different  number  of  times. 

1.  Soil  dried  once 100.00 

2.  Soil  dried  twenty  times  31.44 

3.  Soil  dried  twenty  times 30.60 

4.  Soil  dried  twenty  times  32.05 

Average 31.40 

From  this  table  it  appears  that  the  effect  of  twenty 
times  drying  is  to  reduce  the  force  necessary  to  pene- 
trate the  soil  a  given  uniform  distance  to  one-third  of  that 
for  the  untreated  sample.  This  is  certainly  a  large  change. 

This  fact  has  many  practical  applications.  It  should 
be  observed  that  the  change  in  structure  is  not  associated 
with  continual  wetness,  nor  is  it  any  more  identified 
with  a  continued  dry  state.  In  neither  case  is  the  force 
necessary  to  change  the  structure  brought  to  bear  on 
the  particles.  This  is  exerted  in  the  dri/ing  process. 
It  is  a  well-known  fact  that  soils  which  are  continually 
wet  are  usually  in  bad  physical  condition.  In  the  drain- 
age of  wet  land,  it  is  found  that  the  soil  is  at  first  very 
refractory;  but,  when  good  drainage  is  established, 
there  is  a  gradual  amelioration  of  the  physical  condition 
which  is  primarily  a  change  in  structure.  On  the  other 
hand,  in  a  soil  continually  in  a  dry  state  there  is  no 
change  in  granulation.  The  improvement  of  soil  struc- 
ture, as  a  result  of  changes  in  the  moisture  content, 
is  dependent  largely  on  lines  of  weakness  in  the  soil 
mass.  Some  of  these  are  produced  in  the  process  of 
drying  and  others  in  ways  already  noted. 


108 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


45.  Formation  of  ice  crystals.  —  As  will  be  seen  in 
the  consideration  of  soil  moisture,  the  water  is  distrib- 
uted in  the  fine  pores  in  the  soil.  When  it  freezes, 
it  crystallizes  in  long  needle-like  crystals.  The  crystal- 
lizing force  seems  to  be  considerable.  In  freezing,  the 
crystals  gradually  grow  first  in  the  larger  spaces.  There 

is  a  marked  with- 
drawal of  mois- 
ture from  the 
smallest  spaces 
to  build  up  the 
ice  crystals  in 
the  large  spaces. 
The  soil  mass  is 
separated  by  the 
crystal,  and  the 
result  of  a  single 
hard  freeze  of  a 
wet  soil  is  to 
shatter  it  into 
pieces.  And  the 
repetition  of  this 
process  by  sub- 
sequent freezing 
further  breaks  up 
the  soil,  that  is, 
it  creates  new 
lines  of  weak- 
ness. This  weak- 


Flo. 31.  Ice  crystals  formed  on  the  surface  of 
a  heavy  clay  soil.  These  crystals  are  very  effective 
in  breaking  up  the  soil  and  promote  the  process  of 
granulation. 


ness     is     shown 
by  the  following 


110         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


Fio.  33.  Effect  of  freezing  on  the  granulation  of  clay  soil.  The  lower  pan 
of  soil  on  the  left  was  not  frozen.  That  on  the  right  was  frozen  and  thawed 
once.  The  upper  pan  of  soil  on  the  left  was  frozen  and  thawed  three  times, 
the  one  on  the  right  five  times.  Notice  the  increased  number  of  checks  on  the 
more  frequently  frozen  soil. 

table,  which  represents  the  effect  of  repeated  freezing 
of  a  uniform  sample  of  wet  puddled  clay,  after  which 
all  were  permitted  to  dry  out.  The  figures  are  for  the 
weight  necessary  to  force  a  knife-edge  a  uniform  dis- 
tance into  the  soil,  in  each  case  reduced  to  basis  of  100. 

1.  Check,  unfrozen 100.00 

2.  Frozen  once     .  .  4 30.31 

3.  Frozen  three  times 27.33 

4.  Frozen  five  times   .  .21.88 


TILLAGE  AND  STRUCTURE 


111 


This  process  has  several  interesting  illustrations. 
In  concrete  work,  the  freezing  of  the  material  in  the 
wet  state,  before  it  has  an  opportunity  to  harden,  is 
recognized  as  decidedly  injurious  to  the  strength  of 
the  wall.  The  development  and  action  of  the  ice  crystals 
may  be  readily  observed  in  the  freezing  of  any  thoroughly 
wet  soil  in  winter.  Such  examples  are  shown  in  Figs.  31, 
32  and  33. 

To  a  much  less  extent,  expansion  and  contraction 
of  the  soil  mass  caused  by  variations  in  temperature 
may  contribute  to  the  formation  of  granules.  Any 
movement  of  the  particles  will  tend  to  produce  changes 
in  the  cohesive  forces,  and  when  the  particles  can  move 
easily  they  are  drawn  together  by  this  attraction. 

46.  Tillage. — The  effect  of  tillage  upon  soil  structure 
is  to  produce  lines  of  cleavage,  and  these,  when  produced 
by  plowing,  are  multitudinous,  and  quite  uniformly 
distributed.  As  pointed  out  by  King,  plowing  when 
the  moisture  content  is  suitable  tends  to  break  the 
soil  into  thin  layers,  which  move  one  over  the  other, 


Fid.  34.  Plow  with  interchangeable  moldboard  and  share,  which  adapts 
it  to  different  kinds  of  plowing  The  plow  tends  to  shear  the  soil  into  thin 
layers  which  are  very  thoroughly  broken  up. 


112 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


like  the  leaves  of  a  book,  when  the  pages  are  bent.  This 
disturbance  of  the  existing  arrangement  of  particles 
starts  in  motion  two  forces:  (1)  The  surface  tension 
of  the  water  films,  which  must  now  readapt  themselves 


FIG.  35.     Clay  soil  plowed  when  very  wet.  Condition  indicated  by  the  slickened 
soil  surfaces  and  coarse,  dense  structure 

to  the  new  arrangement,  and  which,  by  opening  larger 
spaces,  may  lose  some  moisture  by  evaporation  into 
the  larger  interstitial  spaces.  (2)  The  cohesive  forces 
between  particles,  some  of  which  have  been  forced 
closer  together  and  some  farther  apart.  The  strength 
of  cohesion  between  small  particles,  like  clay,  can  be 


PLANT   ROOTS   AND   STRUCTURE  113 

realized  when  one  considers  the  tenacity  with  which 
these  particles  are  held  together  in  brick.  This  cohesive 
attraction  is  inversely  proportional  to  the  square  of 
the  distance  between  the  centers  of  the  attracting 
bodies.  Particles  that  can  be  brought  so  closely  to- 
gether as  can  clay  particles  are  thus  held  with  great 
firmness.  The  effect  of  tillage,  when  an  excess  of  water 
is  present,  is  to  force  the  particles  into  large  masses, 
which  become  clods  when  dry.  These  masses  are  too 
large  to  form  granules,  and  leave  the  soil  in  a  compact 
condition,  poorly  adapted  to  plant  growth.  When  the 
soil  is  very  dry  when  worked,  the  particles  are  not 
brought  close  enough  together  to  cohere,  but  are  pow- 
dered, forming  the  separate -grain  structure,  which 
forms  clods  when  wet.  Tillage  may  thus  produce  a 
granular  structure  when  the  moisture  is  neither  excessive 
nor  deficient,  and  the  separate -grain  structure  when 
either  of  these  conditions  exists. 

47.  Growth  of   plant  roots. — The  growth  of    plant- 
roots  changes  the  soil  structure  by  forcing  the  particles 
apart  at  each  growing  root  point,  and  possibly  by  some 
action  yet  to  be  explained.   Crops  differ  greatly  in  their 
effect   upon   soil   structure.     Grass,    millet,    wheat    and 
other  plants  with  fine  roots  are  more  beneficial  to  tilth 
than  coarse  or  tap-rooted  plants  as  corn,  oats  and  beets. 
Grass  also  affects  structure  by  protecting  the  surface 
of  the  ground.    (See  page  119.)    It  is  advisable  to  prac- 
tice a  rotation  on   clay  soil,   which   requires  relatively 
infrequent  plowing,  and  gives  long  periods  in  fine-rooted 
grass  and  grain  crops. 

48.  Organic  matter. — Soils  rich  in  humus  or  decom- 


114 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


posed  organic  matter  are  generally  in  better  physical 
condition  than  soils  low  in  organic  content.  The  marked 
effect  of  the  absence  of  this  material  in  many  long  cul- 
tivated soils  is  well  known.  For  example,  in  much  of 
southern  New  York  the  hill  soils  are  now  recognized 
to  have  a  much  different  relation  to  crop  growth  than 
they  had  for  a  few  years  after  they  were  cleared.  Their 
color  has  changed,  and  with  the  decay  of  the  humus 


Fio.  36.      The  spring-toothed  harrow.      A  type  of  cultivator  adapted    to  all 
classes  of  soil  and  more  efficient  than  any  other  in  rough  and  stony  ground. 

has  come  a  decided  physical  change  in  the  soil,  which 
is  largely  corrected'  by  the  restoration  of  the  humus 
content.  In  certain  prairie  soils  the  effect  of  humus 
depletion  on  structure  is  even  more  marked.  The  actions 
of  humus  are  many,  as  will  be  noted  in  the  more  com- 
plete discussion  of  that  topic  yet  to  follow;  but  one 
of  those  actions  is  on  the  granular  nature  of  the  soil. 
(1)  As  will  appear,  humus  is  somewhat  plastic,  and 
tends  to  hold  the  soil  in  a  more  loose  condition  than 


ORGANIC   MATTER   AND   STRUCTURE  115 

would  otherwise  occur,  and  the  large  spaces  thus  pro- 
duced constitute  lines  of  weakness.  (2)  It  is  a  property 
of  humus  to  undergo  great  change  in  volume  when 
dried  out.  This  is  another  factor  akin  to  the  fineness 
of  the  soil,  and  produces  larger  shrinkage  crack.  This 
is  noticeable  in  many  black  clay  soils,  which  check 
excessively.  (3)  The  great  capacity  of  humus  for 
moisture  permits  a  wide  range  in  moisture  content, 
which  produces  corresponding  physical  alteration.  (4) 
The  color  of  the  humus  affects  the  color  of  the  soil, 
and  thereby  increases  the  rate  of  change  from  wet  to 
the  dry  state  by  increased  evaporation  of  moisture. 
The  relative  effects  of  crude  muck,  and  the  ammonia 
extract  from  the  same  muck,  upon  the  cohesion  of  the 
soil,  as  indicated  by  the  force  required  for  a  uniform 
penetration  of  a  knife-edge  reduced  to  a  basis  of  100, 
is  shown  in  the  following  table.  Samples  dried  and 
rewetted  twenty  times. 

CRUDE   MUCK 

1.  Check 100.00 

2.  Muck,  5  per  cent 82.00 

3.  Muck,  15  per  cent 73.50 

4.  Muck,  25  per  cent 58.48 

5.  Muck,  50  per  cent 50.25 

AMMONIA   EXTRACT  OF  CRUDE   MUCK 

1.  Check 100.00 

2.  Muck  extract,  1  per  cent 85.30 

3.  Muck  extract,  2  per  cent 76.40 

4.  Muck  extract,  4  per  cent 69.00 

This  table  indicates  that   the   material   represented 
by  the  muck  extract  is  the  constituent  of  the  muck 


116         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

which  most  influences  the  structure  of  the  soil.  All 
these  processes  promote  the  development  of  lines  of 
weakness  upon  which  the  water  may  act.  In  this  way 
organic  matter  promotes  granulation. 

49.  Soluble  salts. — In  the  action  of  certain  salts, 
a  different  process  from  the  previous  ones  is  introduced. 
When  lime  is  mixed  with  water  containing  fine  particles 
in  suspension,  there  is  almost  immediately  a  change  in 
the  arrangement  of  the  particles.  They  appear  first 
to  draw  together  in  light  fluffy  groups  or  floccules, 
which  then  rapidly  settle  to  the  bottom,  so  that  the 
supernatant  liquid  is  left  clear  or  nearly  so.  This  phe- 
nomenon is  termed  flocculation,  because  of  the  groups 
of  particles.  It  is  not  an  action  limited  to  lime,  but  in 
greater  or  less  degree  results  from  the  use  of  many 
substances.  Lime  is  about  the  most  active  flocculating 
agent,  and  a  very  small  amount  is  required.  Acids, 
especially  the  mineral  acids,  are  strong  flocculating 
agents.  Many  of  the  common  fertilizing  materials  have 
a  flocculating  power.  Some  substances,  however,  pre- 
vent or  break  up  flocculation.  Such,  for  example,  is  the 
effect  of  carbonates  of  the  alkalies.  From  an  agricul- 
tural point  of  view,  the  various  forms  of  lime  are  the 
most  important  in  this  connection  because  their  use 
on  the  soil  is  practical  for  the  farmer.  When  introduced 
into  the  soil,  this  flocculating  action  occurs  whereby 
granules  are  formed,  the  stability  of  which  may  be 
further  increased  by  other  favoring  conditions. 

The  effect  of  lime  on  this  process  and  the  relative 
rapidity  of  the  action  for  different  forms,  as  shown  by 
its  influence  on  the  cohesion  of  a  puddled  soil,  is  shown 


SOLUBLE   SALTS   AND   STRUCTURE  117 

by  the  following  table.  The  force  required  for  a  uniform 
depth  of  penetration  of  the  knife-edge  is  reduced  to  the 
basis  of  100  for  the  check. 

1.  Check 100.0 

2.  Calcium  carbonate,  5  per  cent 98.5 

3.  Calcium  oxid  equivalent  to  CaO  in  2 56.5 

4.  Calcium  carbonate,  10  per  cent 111.0 

5.  Calcium  oxid  equivalent  to  CaO  in  4 43.5 

G.  Calcium  carbonate,  25  per  cent 05.0 

7.  Calcium  oxid  equivalent  to  CaO  in  6 33.6 

This  table  indicates  that  the  oxide  of  lime,  or  the 
hydrate  as  it  would  be  in  the  wet  soil,  is  more  efficient 
in  granulating  the  soil  than  is  the  carbonate.  It  is  pos- 
sible that  this  difference  is  the  result  of  the  short  time 
of  contact  of  the  lime  with  the  soil,  which  was  only 
a  few  weeks.  In  the  soil  the  hydrate  will  in  time  change 
to  the  carbonate,  owing  to  the  presence  of  carbon 
dioxid  in  the  soil,  so  that  in  the  end  the  form  of  the 
lime  would  be  the  same.  These  figures  emphasize 
a  fact  recognized  in  practice,  viz.,  that  a  considerable 
time  is  necessary  for  lime  to  have  its  full  effect  on  the 
soil,  and  therefore  it  should  be  applied  some  months 
or  even  a  season  or  two  in  advance  of  the  crop  it  is  to 
benefit. 

Warington  reports  the  statement  of  an  English 
farmer  to  the  effect  that  by  the  use  of  large  amounts 
of  lime  on  their  heavy  clay  soil  they  were  enabled  to 
plow  with  two  horses  instead  of  three.  It  is  generally 
true  that  soils  rich  in  lime  are  well  granulated,  and 
maintain  a  much  better  physical  condition  than  soils 
of  the  same  texture  which  are  poor  in  lime. 


118 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


Carbonates  of  the  alkalies,  which  are  present  in 
many  alkali  soils,  tend  to  produce  a  compact  soil  struc- 
ture. The  remedy  lies  in  a  conversion  of  the  carbonate 
into  some  other  form,  or  in  the  removal  of  the  alkali. 


Fio.  37.     A  portion  of  a  Meeker  harrow,  showing  its  effect  on  lumpy, 
clay  soil.   (See  page  481 .) 

(See  page  314.)  Hall  has  shown  that  the  continued 
and  extensive  use  of  nitrate  of  soda  on  the  land  may, 
even  in  humid  regions,  deflocculate  the  soil. 

50.  Animal  life. — Many  forms  of  animal  life  affect 
the  soil  structure.  Earth-worms,  in  passing  soil  through 
their  bodies,  leave  it  in  a  granulated  condition  in  the 


RAINFALL   AND  STRUCTURE  119 

"casts"  which  they  deposit.  Their  action  is  frequently 
quite  important.  (See  page  28.)  Insects,  especially 
ants  and  other  burrowing  creatures,  aid  in  this  and 
other  ways. 

61.  Rainfall. — Rain  storms  compact  the  surface  soil 
by  washing  the  fine  particles  into  the  interstitial  spaces, 
and  by  the  actual  pressure  of  the  rain-drops.  The  result 
is  to  form  a  surface  layer  having  the  separate-grain 
structure,  and  which  when  dry,  forms  a  crust,  some- 
times capable  of  preventing  germinating  plants  from 
reaching  the  surface  of  the  ground,  and  which  is  con- 
ducive to  the  loss  of  moisture  by  evaporation.  Some 
clay  soils  are  very  susceptible  to  a  change  of  structure 
in  this  way.  A  heavy  thunder-storm  may  entirely 
change  the  structure  of  a  clay  soil  to  the  depth  of  sev- 
eral inches,  in  a  short  time.  This  is  due  both  to  the  im- 
pact of  the  rain-drops  and  the  saturated  condition  of 
the  surface  layer. 

Surface  covering — mulches,  sod  or  any  kind  of 
covering  during  the  summer  season — serves  to  protect 
the  surface  soil  from  the  compacting  effect  of  rain.  The 
volume  weight  of  a  mulched  soil  will  generally  be  found 
to  be  less,  at  the  end  of  the  growing  season,  than  that 
of  a  well-cultivated  one.  The  benefit  to  be  derived 
from  sod  has  already  been  mentioned.  A 

i 

II.     ORGANIC    CONSTITUENTS    OF    THE    SOIL 

Examination  of  almost  any  soil  shows  it  to  contain 
not  only  mineral  particles  but  also  plant  and  animal 
remains.  The  forest  soil  contains  in  the  surface  layer 


120         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

a  large  amount  of  partially  decayed  leaves  and  stems, 
sometimes  termed  leaf-mold.  Sod  land  is  filled  with 
fine  roots,  which  have  served  their  period  of  usefulness 
to  the  plant  and  are  being  returned  to  their  native 
elements.  The  low  swamp  areas  of  soil  contain  a  large 
proportion  of  dark  or  black  material,  which,  when  the 
soil  is  dry,  may  be  burned  away,  and  leaves  the  residue 
a  much  lighter  color.  And  in  all  soils  there  is  some  of 
this  same  volatile  material  derived  from  the  growth 
and  partial  decay  of  plants  and  animals,  and  commonly 
termed  organic — organized  matter. 

Organic  matter  may  be  found  in  the  soil  in  all  stages 
of  decay,  from  the  fresh  tissues  to  the  last  oxidation 
products  of  its  components.  These  products  of  the  var- 
ious stages  in  the  decay  process,  comprehended  by  the 
term  organic  matter,  constitute  probably  the  most 
important  body  of  material  which  enters  into  normal 
soil. 

62.  Sources,  derivation  and  forms. — The  organic 
matter  in  the  soil  is  derived  from  both  plants  and 
animals:  plants  are  the  chief  source.  These  materials 
undergo  decay  through  the  action  of  bacteria  and 
fungi,  in  addition  to  purely  chemical  changes.  The 
character  of  the  intermediate  material  depends  largely 
on  the  relative  prominence  of  the  different  agencies 
concerned  in  its  decomposition.  This  great  variety 
in  the  material,  together  with  the  differences  in  pro- 
cesses of  decay,  gives  rise  to  a  number  of  forms  of  or- 
ganic material  which  are  recognized  in  the  soil.  These 
materials  do  not  represent  any  definite  composition. 
They  represent,  rather,  stages  in  the  general  process 


ORGANIC    MATTER.     COMPOSITION  121 

of  decay.  Leaf-mold  is  the  partially  decomposed  layer 
of  leaves,  twigs,  etc.,  found  on  the  surface  of  the  ground, 
usually  in  well-drained  forest  areas.  Decomposition 
is  very  incomplete.  Humus  is  the  black  or  brown  pul- 
verent  material  resulting  from  a  considerably  more 
advanced  stage  of  decay  than  is  represented  by  leaf- 
mold.  When  wet,  it  forms  a  very  fine,  gelatinous  mass 
of  a  colloidal  nature.  Peat  represents  large  and  usually 
deep  accumulations  of  plant  remains  in  the  early 
stages  of  decay.  Disintegration  has  usually  been  stopped 
by  the  saturation  of  the  mass.  The  products  of  bacterial 
and  fungicidal  action  have  accumulated,  until  the 
organisms  are  killed,  and  any  further  growth  is  pre- 
vented until  the  water  is  removed  and  more  thorough 
aeration  is  introduced.  Plant  tissues  are  plainly  evident. 
When  the  peat  results  from  a  particular  kind  of  plant, 
the  name  of  the  latter  may  be  affixed  as  moss  peat. 
Peat  is  generally  unproductive  as  a  soil.  Muck  repre- 
sents a  much  more  advanced  stage  in  the  decay  of  peat. 
It  has  a  black  or  brown  color,  more  closely  resembling 
humus,  due  to  the  large  proportion  of  the  latter  which 
it  contains.  Plant  tissues  are  much  less  apparent.  It 
is  generally  productive,  or  will  very  quickly  become  so 
under  drainage  and  cultivation. 

53.  Chemical  composition. — There  is  no  definite 
chemical  composition  of  the  organic  matter  in  the  soil. 
It  is  as  variable  as  the  materials  from  which  it  is  derived 
and  the  conditions  under  which  it  is  formed.  It  is 
composed  of  a  great  variety  of  carbon  compounds, 
into  which  enter  nitrogen  and  all  of  the  mineral  ele- 
ments which  are  necessary  to  plant  and  animal  growth. 


122         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

These  original  compounds  are  broken  down  in  the  pro- 
cess of  decay  into  other  successively  simpler  com- 
pounds. The  end  of  the  process  is  always  essentially 
the  same — the  reduction  of  the  elements  to  their  sim- 
plest and  most  stable  forms,  the  carbon  to  carbon  dioxid, 
the  nitrogen  to  nitrates,  ammonia  or  even  free  nitrogen, 
and  the  mineral  elements  to  their  simple  salts.  The 
soil  constituents  which  are  termed  humus,  mold,  peat, 
muck,  etc.,  simply  represent  stages  in  the  transition 
process  from  the  fresh  materials  to  the  native  elements. 
There  is  no  single  compound  or  group  of  compounds 
which  imparts  definite  characteristics.  These  are  the 
result  of  the  mixture;  and  this  fact  of  an  infinitely 
complex  mixture  is  exceedingly  important  to  keep  in 
mind,  in  considering  the  effects  of  the  organic  matter 
of  the  soil.  Many  of  them  are  acids.  Some — as  ammonia 
and  marsh  gas — function  as  bases.  They  react  with 
each  other  in  many  ways,  and,  what  is  more  important, 
they  react  with  the  mineral  elements  of  the  soil  to  form 
organic  salts.  It  is  by  this  union  that  organic  matter 
has  not  only  a  direct  effect  as  a  food,  but  also  an  indirect 
effect  in  releasing  food  elements  from  their  less  soluble 
mineral  combinations.  Aside  from  the  production  of 
many  complex  organic  acids,  the  two  most  significant 
facts  of  their  composition  are  the  per  cent  of  nitrogen 
present  and  the  chemical  form  of  part  of  the  carbon. 
Nitrogen,  which  is  not  a  constituent  of  rocks,  is  made 
available  to  all  higher  forms  of  plants  through  this  or- 
ganic decay  process,  and  these  various  compounds 
constitute  the  soil  store-house  of  the  element  from  which 
it  gradually  changes  over  into  the  available  forms.  The 


NITROGEN   IN   ORGANIC   MATTER  123 

percentage  of  nitrogen  present  varies  greatly — viz,  from 
less  than  2  per  cent  in  the  humus  of  some  humid  soils 
to  more  than  22  per  cent  in  the  humus  of  some  arid 
soils,  as  reported  by  Hilgard.  His  results  show  that 
under  arid  and  semi-arid  conditions  the  humus  is  much 
more  rich  in  nitrogen  than  in  humid  regions,  and  he 
attributes  to  this  fact  the  large  capacity  of  the  former 
soils  to  produce  crops  with  so  little  organic  matter. 
His  figures  on  this  point  are  exhibited  in  the  following 
table. 

PER  CENT  OF  NITROGEN   IN   HUMUS  OF  SOIL  FROM 
DIFFERENT   REGIONS 

Humid  soils,  average  of  sixteen  samples 4.58 

Sub-irrigated  arid  soils,  average  of  fifteen  samples    .  .   8.38 
Arid  upland  soils,  average  of  forty-two  samples 15.23 

The  nitrogen  is  changed  under  good  soil  conditions 
to  forms  available  to  plants. 

There  is  a  similar  relative  increase  in  the  proportion 
of  carbon  in  humus  over  that  in  the  original  material. 
The  coals  are  metamorphosed  muck  and  peat  deposits, 
and  their  value  for  fuel  lies  in  their  carbon  content. 
Hilgard  has  shown  by  a  series  of  analyses  that  there 
is  a  gradual  increase  in  the  carbon  content  during  the 
decay  process,  at  least  up  to  the  humus  stage,  which 
is  shown  physically  by  the  darkening  of  the  material. 
This  darkening,  which  appears  in  peat  and  muck  may 
be  the  result  of  the  separation  of  free  carbon  which,  in 
the  amorphous  form,  is  black.  Its  practical  significance 
in  a  soil  way  is  its  large  effect  on  the  color  of  the  soil, 
which  alters  its  heat  relations.  Crops  always  start  first 


124         THE   PRINCIPLES  OF  SOIL   MANAGEMENT 

on  black  soils,  other  things  equal,  and,  as  has  been 
stated,  this  dark  color  is  generally  due  to  humus. 

54.  Amounts  present. — The  amount  of  organic  matter 
present  varies  greatly  with  different  soils.  Peat  and 
muck  deposits  are  very  largely  organic  material,  the 
per  cent  depending  on  the  state  of  decomposition. 
Some  porous,  well-drained  soils  are  almost  lacking  in 
this  constituent.  But  nearly  all  soils  have  a  moderate 
per  cent.  The  accumulation  is  larger  in  the  soil  than  in 
the  subsoil,  and  generally  decreases  with  depth.  In 
237  types  of  soil,  representing  thousands  of  samples 
from  all  parts  of  the  United  States,  the  soil  was  found 
to  contain  2.06  per  cent,  and  the  subsoil  .83  per  cent. 
This  latter  refers  to  the  upper  subsoil,  and  at  greater 
depths  the  organic  content  is  very  much  less.  But  in 
those  soils  recently  formed  by  stream  action  the  organic 
content  in  the  third,  fourth  and  fifth  foot  may  be  very 
considerable,  as  is  indicated  by  the  color. 

In  general,  arid  soils  contain  less  organic  matter 
than  soils  of  humid  regions ;  those  of  cold  climates 
more  than  those  of  warm  climates.  The  soils  of  the 
northern  states  and  Canada  are  very  generally  quite 
dark  colored,  while  those  of  the  southern  states  under 
similar  treatment  are  much  lighter  colored,  due  to  dif- 
erence  in  organic  content.  Wet  soils  contain  more  than 
dry  soils,  and  clay  soils  contain  more  than  sandy  soils. 

These  facts  are  illustrated  by  the  following  figures, 
showing  the  amount  of  organic  matter  in  different  soils, 
which  in  the  first  six  lines  are  the  average  of  ten  samples 
representing  several  soil  types  of  approximately  the 
same  natural  drainage. 


AMOUNT   OF   ORGANIC   MATTER    IN   SOIL  125 


TABLE  XVII 


Sandy  soils 

Loam  and  clay  loam 
soils 

Soil 
Per  cent 
organic 
matter 

Subsoil 
Per  cent 
organic 
matter 

Soil 
Per  cent 
organic 
matter 

Subsoil 
Per  cent 
organic 
matter 

Northeastern  states    

1.66 
0.93 
1.84 
1.16 
0.99 
0.89 

0.60 
0.41 
0.76 
0.55 
0.62 
0.64 

3.73 
1.53 
3.06 
1.80 
2.64 
1.05 

1.35 
0.73 
1.07 
0.65 
1.11 
0.62 

Southeastern  states  

North  Central  states  

South  Central  States    .... 
Semi-arid  states  

Arid  states.  . 

Soil  0-7  inches    . 
Per  cent 
organic  matter 

Subsoil  7-40  inches 
Per  cent 
organic  matter 

Illinois  deep  peat  and  muck  
Miami   black   clay  loam,  average 
twelve  samples.             .  . 

84.6 
5.9 

55.80 
2.50 

Portsmouth  sandy  loam,  average 
nine  samples. 

4.1 

0.92 

Wabash  silt  loam,  average  eleven 
samples  

3.3 

1.30 

The  Miami  black  clay  loam  is  a  famous  corn  soil  of 
the  North  Central  states,  and  comprises  areas  of  glacial 
clay  loam,  which  were  originally  very  wet  and  swampy, 
but  have  been  reclaimed  by  drainage.  The  Portsmouth 
sandy  loam  occurs  in  the  coastal  plain  of  the  southern 
states,  and  represents  a  mild  form  of  swamp  soil,  but 
in  which  the  accumulation  of  organic  matter  is  not 
sufficient  to  permit  its  classification  as  muck.  When 
well-drained,  this  soil  is  a  first-class  truck  soil.  The 


126         THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

Wabash  silt  loam  is  the  much-prized  deep,  dark  silt 
loam  of  the  stream  bottoms  in  the  North  Central  states. 
It  is  a  close  competitor  of  the  Miami  black  clay  in  the 

production    of    corn 
and  grass. 

The  proportion  of 
organic  matter  is  the 
chief  distinction  be- 
tween soils  and  sub- 
soils. It  will  be  noted 
from  the  table  that 
in  all  of  the  humid 
sections  this  differ- 
ence is  very  marked, 
and  agrees  well  with 
the  color  differences 
generally  observed. 
But  in  the  arid  re- 
gions this  distinction 
between  soil  and  sub- 
soil is  not  so  obvious. 
The  difference  in  or- 
ganic content  is  even 

Fio.  38.    A  soil  of  loamy  texture  in  good  tilth.         ,  ,       .     ,  , 

less  marked  than  the 

figures  indicate,  for  the  reason  that  several  of  those 
for  the  soil  extend  to  a  depth  of  two  feet  or  more  and 
those  of  the  subsoil  extend  often  to  six  feet. 

56.  Some  physical  properties. — The  physical  prop- 
erties of  the  organic  constituents  of  the  soil  are  different 
in  value  from  those  of  the  mineral  constituents.  Though 
usually  present  in  small  amounts  their  properties 


PH YSICA  L  PROPERTIES  OF  ORGA NIC  MA  TTER       127 

are  such   as  to   have  a  large  influence  on  its  produc- 
tiveness. 

66.  Solubility. — The  organic  matter  may  be  divided 
into  two  general  classes  of  materials:  (a)  If  an  ordinary 
soil  or  peat  or  muck  be  leached  with  water,  particularly 
if  the  water  contain  a  little  ammonia,  a  dark  brown 
or  black  color  will  be  imparted  to  the  extract.  This 
is  due  to  a  mixture  of  organic  compounds  which  have 
a  colloidal  or  gelatinous  consistency.  It  is  the  material 
— to  which  the  specific  term  humus  is  applied — which 
gives  the  brown  color  to  the  drainage  water  from 
swamps  and  to  the  teachings  from  the  manure  heap. 
This  color  is  an  indication  of  the  loss  of  the  humus 
constituent,  and  should  remind  one  of  the  necessity 
for  precautions  against  the  loss,  as  far  as  possible.  When 
the  humus  is  united  with  salts  like  lime  to  form  humates, 
this  loss  is  very  much  reduced.  It  follows  from  this 
that  the  loss  of  humus,  by  leaching  from  soils  rich  in 
lime,  is  very  much  less  than  in  those  soils  poor  in  lime. 
Many  of  the  soils  in  the  southern  states  are  very  low 
in  lime,  and  the  streams  are  generally  bordered  by 
swampy  areas.  As  a  result,  the  drainage  water  is  usually 
of  a  brown  coffee-color.  On  the  other  hand,  in  those 
northern  states  where  the  soils  are  rich  in  lime,  this 
brown  color  is  much  less  pronounced  and  is  usually 
absent.  If  lime  or  some  other  flocculating  agent  be 
added  to  this  brown  liquid,  the  humus  separates  out 
in  fluffy  masses,  which  settle  to  the  bottom,  leaving 
the  liquid  above  almost  colorless.  This  is,  in  part,  what 
takes  place  in  the  soil  when  these  flocculating  materials 
are  present. 


128         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

(6)  The  remainder  of  the  organic  material,  after 
extraction,  is  composed  of  the  fresh  and  partially 
decomposed  fragments  of  plant  and  animal  remains 
more  or  less  stained.  It  is  a  light  chaffy  material,  which 
by  decay  may  be  changed  to  humus,  but  in  this  condition 
is  riot  subject  to  direct  loss. 

67.  Weight. — The  organic  material  is  the  lightest 
constituent  in  the  soil.  Warington  gives  the  specific 
gravity  of  humus  as  1.2  to  1.5,  as  compared  with  2.68 
for  the  mineral  constituents,  and  Hilgard  reports  its 
volume  weight  when  dry  as  .33,  as  compared  with 
about  1.1  for  clay  and  1.5  for  sand.  Therefore,  in  pro- 
portion as  a  soil  contains  humus,  it  is  lighter  in  weight. 
On  the  basis  of  the  above  figures,  a  cubic  foot  of  humus 
would  weigh  about  twenty-one  pounds.  Muck  and  peat, 
however,  contain  mineral  matter  washed  in  with  the 
organic  material  and  their  volume  weight  is  higher. 
It  ranges  from  twenty  to  forty-five  pounds  per  cubic 
foot,  according  to  the  stage  of  decay  when  dry. 

58.  Absorption  properties. — In  the  form  of  humus, 
organic  matter  has  a  very  large  absorptive  power  for 
gases  and  salts  in  solution,  similar  to  that  shown  by 
powdered  charcoal.  It  is  much  greater  than  that  of 
even  clay  soils,  and  for  this  reason  its  addition  to  soil 
increases  this  important  property. 

69.  Volume  changes. — Like  clay  soil,  when  humus 
is  dried,  it  shrinks  very  greatly,  and  conversely,  when 
it  is  moistened  it  expands.  In  humus  this  property 
is  much  more  pronounced  than  in  even  the  heaviest 
clay.  Warington  reports  the  shrinkage  of  a  very  pure 
clay,  in  drying  from  a  saturated  state,  to  be  18  per  cent 


EFFECTS   OF   ORGANIC   MATTER  129 

of  the  original  volume,  and  that  of  humus  to  be  20  per 
cent;  while  others  report  the  shrinkage  of  muck  samples 
to  be  more  than  twice  this  amount. 

60.  Plasticity. — The  crude  organic  matter  exhibits  no 
striking    peculiarities,    but    the    humus    substance    has 
many.    One  of  these  is  its  plasticity.     Although  very 
fine,  its  plasticity  is  not  great  as  compared  with  clay. 
But  it  is  sufficient  to  act  as  a  weak  cementing  material 
in  soil,   which   is   very   important   in   binding  together 
light  sandy  soils,  and  in  lightening  up  and  holding  apart 
the  aggregates  or  crumbs  in  clay  soil.   Thereby  it  greatly 
promotes  the  granulation  of  clay  soils  which  are  properly 
drained. 

61.  Effects  of  organic  matter. — The  effects  of  organic 
matter   on   the  soil,    and   thereby   upon   plant    growth, 
are  so  numerous  and   so  far-reaching  and  generally  so 
beneficial,  and  further,  its  maintenance  is  so  important 
a  part  of  good  soil   management  that,   at   the  risk  of 
anticipating    some  of    the   subsequent    discussions,   its 
effects  are  here  briefly  summarized.    They  are  of  two 
sorts,  (1)   Physical,  and  (2)  Chemical. 

62.  Physical  effects.— (a)   Physically,  it   affects  both 
tilth    and    granulation.     Owing    to    its    weak    plasticity 
and  its  great  contraction  when  dried,  it  is  a  very  potent 
factor    in    hastening    the    granulation    process    of    clay 
soils  in  the  way  that  has  been  explained  above.    And 
on  light  sandy  soils  which  are  loose  and  inclined  to  be 
drifted  by  the  wind  or  eroded  by  rains,  it  has  the  effect 
of  binding  them   together  and   imparts   a   much   more 
loamy  character. 

(6)  By  its  beneficial  effect  on  the  structure  of  the 


130         THE   PRINCIPLES  OF  SOIL*  MANAGEMENT 

soil,  it  very  greatly  increases  its  moisture-holding 
capacity,  which  is  further  increased  by  the  great  ca- 
pacity of  humus  itself  to  retain  water,  which  amounts 
to  200  or  300  per  cent  of  its  dry  weight,  as  compared 
with  10  or  15  per  cent  for  sarfdy  loam  and  25  to  35  per 
cent  for  clay  soils.  It  therefore  improves  the  drought 


FIG.  39.      The  solid  disc  harrow.     Most  efficient  on  medium  heavy  soil  free 
from  stone  and  rubbish. 

resistance    of    soils    by    increasing    their    reservoir    for 
available  water. 

(c)  The  dark  color  which  humus  imparts  to  soils 
permits  them  to  absorb  the  heat  of  the  sun's  rays  very 
much  more  than  when  the  humus  is  absent,  and  thereby 
their  average  temperature  is  decidedly  raised.  It  is  for 
this  reason  that  the  corn  first  appears  in  the  spring  in 
the  low  areas  of  dark-colored  soil,  which  difference 
in  time  may  amount  to  several  days. 


MAINTENANCE   OF   ORGANIC   MATTER  131 

63.  Chemical  effects. — The   chemical    effects  are   of 
two  sorts:  (a)  Vegetable   and   animal   remains  contain 
all  of  the  essential  elements  of  plant  food,  and  by  their 
decay  these  are  given  back  to  the  soil  in  a  form  readily 
available  as  food  for  other  plants.     It  is  therefore   a 
direct  source  of  food  elements. 

(6)  The  products  of  the  decay  of  organic  matter  are 
many  forms  of  organic  acids,  the  simplest  and  most 
abundant  of  which  is  carbon  dioxid.  In  the  soil  moisture 
these  act  powerfully  upon  the  mineral  soil  particles 
to  bring  their  elements — particularly  the  bases — into 
solution.  Because  of  their  presence,  the  soil  water 
must  be  regarded  as  a  weak  solution  of  all  of  these 
products  and  by  their  presence  its  dissolving  power  is 
greatly  increased. 

64.  Maintenance  of  organic  matter. — Two  conditions 
are    necessary    to    maintain    an    adequate    amount    of 
organic  matter  in  the  soil.   These  are,  first,  an  adequate 
supply,    and   second,    avoidance   of    a    too -rapid    loss, 
together  with  the  maintenance  of  those  soil  conditions 
which  promote  the  proper  form  of  decay. 

The  organic  matter  derived  from  the  higher  plants 
is  supplemented  by  that  from  bacteria  and  fungi,  which 
are  generally  abundant  in  the  soil.  Much  may  be  accom- 
plished by  good  soil  management  to  favor  the  develop- 
ment of  the  lower  forms,  so  that  they  may  be  a  very 
important  source  of  humus.  In  fact,  it  has  been  sug- 
gested that  they  may  sometimes  be  the  chief  source 
of  supply. 

Any  plant  may  be  used  as  a  green  manure,  to  furnish 
organic  matter  to  the  soil.  Plants  which  have  been 


132         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

much  used  for  this  purpose  are  the  clovers,  vetch, 
field-peas,  cowpeas,  soy  beans,  rye,  and  buckwheat. 
When  any  of  these  crops  are  planted  in  the  late  summer 
to  conserve  plant  food,  they  are  termed  "catch  crops," 
and  when  used  to  cover  the  ground  and  protect  it  from 
erosion,  they  are  termed  "cover  crops."  Many  forms 
of  organic  manures  and  waste  materials  are  applied 
as  a  source  of  humus. 

Good  tillage  and  the  proper  rotation  of  crops  greatly 
assist  the  accumulation  of  organic  matter  in  the  soil, 
and  to  these  may  sometimes  be  added  amendments 
such  as  lime.  Some  of  the  conditions  which  favor  the 
accumulation  of  organic  matter  in  the  soil  are:  (1)  The 
presence  of  an  excess  of  water.  (2)  Low  temperature. 
(3)  Limited  aeration.  (4)  Deficiency  of  basic  elements. 
(5)  Absence  of  decay  organisms.  (6)  Application  of 
organic  manures.  (7)  Accumulation  of  plant  residues 
in  the  soil.  (8)  Proper  rotation  of  crops.  (9)  Absence 
of  tillage. 

Some  of  the  conditions  which  favor  the  rapid  dis- 
appearance of  humus  from  the  soil  are:  (1)  The  presence 
of  a  moderate  amount  of  water.  (2)  Thorough  aeration. 
(3)  High  temperature,— from  75°  to  110°  Fahr.  (4) 
Abundance  of  available  basic  elements.  (5)  Abundance 
of  decay  organisms.  (6)  Failure  to  maintain  the  supply 
of  organic  matter.  (7)  Complete  removal  of  all  crops. 
(8)  Improper  crop  rotation.  (9)  Excessive  tillage. 

Good  management  seeks  to  adjust  these  two  sets  of 
conditions,  so  that  large  crops  are  produced  without 
imparing  the  humus  supply  in  the  soil. 


B.   THE  SOIL  AS  A  RESERVOIR  FOR  WATER 

I.     FUNCTIONS    IN    PLANT    GROWTH 

When  plants  grow,  they  use  water.  It  circulates 
through  their  vessels,  is  built  into  their  tissues,  and  is 
evaporated  by  the  leaves.  In  these  capacities  it  per- 
forms three  important  and  vital  functions  for  the  plant. 
It  is  (a)  a  direct  food  of  the  plant,  and  becomes  a  part 
of  its  tissues  either  directly  as  water,  or  it  is  broken 
up  and  its  elements  are  used  in  new  compounds,  (b) 
It  is  a  carrier  of  food  to  the  plant,  and  serves  as  the 
medium  of  transfer  for  the  mineral  elements  from  the 
soil  and  the  gaseous  elements  from  the  air  to  their 
appropriate  points  of  assimilation  and  use  in  the  growth 
of  the  plant  mechanism,  (c)  In  addition  to  the  last  two 
functions,  water  serves  as  a  regulator  of  the  physical 
condition  of  the  plant.  It  equalizes  the  temperature 
of  the  plant  and  modifies  its  stability. 

From  60  to  more  than  95  per  cent  of  the  green  weight 
of  the  staple  crops  is  due  to  water. 

In  the  ordinary  processes  of  growth,  the  amount  of 
water  transpired  is  many  times  greater  than  that  used 
directly  as  food.  Investigations  in  different  parts  of 
the  world  have  shown  that  for  the  production  of  each 
pound  of  dry  matter  ordinary  crops  transpire  from 
200  to  500  pounds  of  water. 

Warington  has  compiled  the  following  figures,  show- 
ing the  amount  of  water  used  by  different  crops  in  the 
production  of  organic  matter. 

(133) 


134         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


TABLE  XVIII. — WATER  EVAPORATED  BY  GROWING  PLANTS  FOR 
ONE  PART  OF  DRY  MATTER  PRODUCED 


Lawes  and  Gilbert 
England 

Hellriegel 
Germany 

Wollny 
Germany 

King 
W  isconsin 

Beans 

214 
225 
235 
249 
262 

Beans 

262 
359 
292 
330 
310 
402 
371 
373 
377 

Maize  

233 
416 

479 
912 
774 
665 
664 
843 
490 

Maize 

272 
423 
447 
453 
393 
557 

Wheat      .    . 

Wheat  .... 

Millet  

Potatoes  .  .  . 
Peas  

Peas  . 

Peas  

Peas  

Red  clover.  . 
Barley 

Red  clover  . 
Barley 

Rape  

Red  clover  . 
Barley   .... 

Barley 

Oats  

Oats  

Oats  

Buckwheat  . 
Lupine  .... 
Rye  .. 

Buckwheat  . 
Mustard  .  .  . 
Sunflower  .  . 

The  variation  exhibited  by  the  figures  for  the  crop, 
as  well  as  for  different  crops,  illustrates  the  influence 
of  climate  and  soil  upon  transpiration.  Other  things 
equal,  more  water  will  be  required  in  an  arid  region 
than  in  one  of  humid  climate;  more  in  a  warm  region 
than  in  a  cold  region;  more  on  a  clay  soil  than  on  a 
sandy  soil;  more  in  a  windy  section  than  in  a  region 
of  still  atmosphere;  more  with  a  high  soil  moisture 
content;  more  on  a  poor  soil;  and,  finally,  more  water 
is  used  per  pound  of  dry  matter  produced  in  a  small 
crop  than  is  required  in  a  large  crop.  All  of  these  figures 
agree  in  indicating  the  large  amount  of  water  used 
in  the  production  of  crops.  Not  only  is  the  total  seasonal 
requirement  to  be  considered,  but  the  maximum  de- 
mands of  the  crop  at  any  period  of  its  growth  must 
be  met.  King  observed  that  a  single  corn  plant  during 
the  first  week  of  August,  when  it  was  coming  into  tassel 
and  the  ear  was  forming,  used  water  at  the  rate  of  1,320 


WATER   REQUIRED   BY   CROPS  135 

grams  (one  and  one-half  quarts)  per  day.  Hunt  observed 
in  Illinois  that  in  one  week  in  July  the  growth  of  corn 
amounted  to  1,300  pounds  of  dry  matter  per  acre.  As- 
suming the  requirement  observed  in  Wisconsin, — 272 
pounds  per  pound  of  dry  matter, — this  is  equivalent 
to  1.55  inches  of  water. 

Assuming    the    average    production    of    dry    matter 
to  be  two  tons  per  acre,  the  amount  of  water  required 


FIG.  40.     Solid,  metal  roller.   The  prevailing  type  of  compacter. 

to  produce  such  a  yield  of  the  staple  crops,  under  the 
best  conditions  of  management,  would  amount,  accord- 
ing to  the  above  figures,  to  from  427  tons  to  1,820  tons 
of  water  per  acre,  which  is  equivalent  to  a  rainfall  of 
3.7  and  15  inches,  respectively. 


II.     AMOUNT    OF    WATER    IN    THE    SOIL 

Soils  exhibit  great  differences  in  moisture  content 
and  in  their  ability  to  meet  the  needs  of  the  plants  for 
water.  In  some  of  the  southeastern  states,  where  the 


136          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

rainfall  is  from  fifty  to  sixty  inches,  crops  suffer  more 
from  a  lack  of  moisture  than  they  do  in  some  of  the 
states  of  the  northern  Mississippi  valley,  with  only 
a  third  of  the  rainfall.  The  light  truck  soils  of  the  At- 
lantic coast  suffer  much  more  from  a  lack  of  water 
than  do  the  interior  soils  of  heavy  texture  which  are 
under  the  same  rainfall  and  general  temperature  con- 
ditions. Plants  in  a  dry  greenhouse  use  more  water  than 
in  the  more  moist  outside  air.  These  illustrations  serve 
to  emphasize  the  three  factors  which  determine  the 
amount  of  moisture  a  soil  contains.  These  are  (a)  the 
available  supply  of  water;  (6)  the  retentive  capacity 
of  the  soil  for  water;  (c)  the  rate  and  amount  of  loss  of 
water  from  the  soil.  Each  of  these  factors  depends 
on  many  conditions. 

65.  The  supply. — The  supply  of  water  is  obviously 
controlled  by  conditions  external  to  the  soil.    These  are 
the  precipitation  in  the  forms  of  rain  and  snow,  under- 
ground seepage,  and  irrigation. 

66.  Retentive   capacity  of    the  soil. — The  retentive 
capacity   of    the  soil    varies    greatly  according   to   its 
physical  properties,   As  soils  ordinarily  occur  in  the- field, 
they   show   the   presence   of   moisture.     This    moisture^ 
is  held  quite  intimately.    Two  soils  may  appear  equally 
moist,  yet  have  very  different  capacities  to  maintain 
crops.     Plants    suffer    much    more    quickly    from    dry 
weather  on  sand  soil  than  on  clay  soil,  even  when  the 
soils  appear  equally  wet  at  the  outset. 

67.  Statement    of    water    content. — Five     different 
methods  are  commonly  used  in  stating  the   moisture 
content  of  soils.    These  are:  (1)  In  terms  of  per  cent 


3i?=»  "o  "o  "o  o  "o  'o  b 

S          "CMO^IOffl 


138          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

based  on  the  dry  weight  of  the  soil.  (2)  In  terms  of 
per  cent  based  on  the  wet  weight  of  the  soil.  (3)  In 
terms  of  the  per  cent  of  volume  based  on  the  total 
volume  occupied  by  the  soil.  (4)  In  cubic  inches  per 
cubic  foot,  or  in  cubic  centimeters  per  liter  or  per  cubic 
meter.  (5)  In  inches  in  depth  of  water  over  the  surface 
of  the  soil. 

Of  these  methods  the  first  is  most  largely  used, 
because  it  gives  the  most  definite  and  constant  basis 
from  which  to  derive  any  other  quantities.  The  dry 
weight  of  a  soil  remains  constant,  and  percentages 
referred  to  that  base  are  always  comparable.  But  it 
has  several  disadvantages  which  lead  to  inconsistent 
results  in  practical  work.  For  example,  10  per  cent  of 
water  in  a  cubic  foot  of  clay  soil  represents  a  very 
different  quantity  of  water  from  the  same  percentage 
in  a  sand  or  a  muck  soil,  because  of  the  very  different 
volume  weights  of 'these  materials.  In  the  clay  it  would 
mean  about  7  pounds,  or  3.5  liters;  in  the  sand  soil 
10  pounds  or  4.5  liters;  and  in  the  muck  soil  3.5  pounds, 
or  1.6  liters, — manifestly  very  different  quantities  of 
water.  Or,  to  state  the  matter  in  a  different  way,  30 
per  cent  of  water  in  a  clay,  12  per  cent  in  sand,  and 
150  per  cent  in  muck,  do  not  represent  as  different 
volumes  of  water  as  is  indicated  by  the  figures,  because 
of  the  relative  weights  of  the  soils.  But,  because  almost 
any  other  figure  can  be  readily  derived  from  the  moisture 
percentage  expressed  in  terms  of  dry  weight  of  soil, 
it  has  been  very  generally  used,  especially  in  laboratory 
studies.  In  field  practice,  a  volume  method  is  more 
convenient. 


STATEMENT   OF   SOIL   MOISTURE   CONTENT        139 

The  second  method — that  based  on  the  wet  weight 
of  the  soil — is  unsatisfactory,  because  it  is  not  only 
open  to  the  objections  made  to  the  first  method,  but 
also  because  figures  on  moisture  content  of  the  same 
sample  of  soil  are  not  comparable.  They  do  not  repre- 
sent the  same  degree  of  wetness  indicated  by  the  per- 
centages. For  example,  100  grams  of  wet  clay  contain- 
ing 10  per  cent  of  water  would  consist  of  10  grams  of 
water  in  90  grams  of  soil,  and  100  grams  of  wet  clay 
containing  20  per  cent  would  consist  of  20  grams  of 
water  in  80  grams  of  soil.  In  the  first  case,  the  ratio 
of  water  to  soil  is  as  1  to  9;  while,  in  the  second,  case 
the  ratio  is  1  to  4,  instead  of  1  to  4.5,  as  the  percentage 
comparison  would  indicate.  The  difficulty  in  deriving 
other  figures  from  percentages  based  on  wet  weight 
makes  its  use  undesirable. 

The  third  method,  statement  of  percentage  of  water 
by  volume,  is  the  most  rational  of  the  first  three.  It 
gives  a  direct  practical  basis  of  comparison  for  all  soils. 
It  shows  the  volume  of  water  held  by  the  soil,  which 
is  really  the  important  consideration  from  the  point 
of  view  of  the  plant.  For  purposes  of  comparing  the 
moisture  content  of  different  soils  in  the  field,  it  is 
probably  the  most  satisfactory  method.  Derivation 
of  these  quantities  involves  considerable  calculation, 
and  often  the  determination  of  some  quantities  not 
readily  obtainable. 

The  fourth  method  of  statement  is  really  a  variation 
in  detail  from  the  third  method  by  which  specific  quan- 
titive  statements  are  made.  One  hundred  seventy-two 
and  eiu;ht-tenths  cubic  inches  of  water  in  one  cubic. 


140         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

foot  of  soil,  is  a  cumbersome  method  of  saying  the  soil 
contains  10  per  cent  of  water  by  volume. 

The  fifth  method  is  most  generally  used  in  field  prac- 
tice in  stating  quantities  of  water.  In  irrigation  practice, 
water  is  often  measured  in  inches  in  depth  per  acre  of 
area.  In  stating  the  quantity  of  water  held  within  root 
range  by  different  soils,  this  method  is  also  direct  and 
convenient.  For  example,  a  sand  soil  of  a  certain  tex- 


Fio.  42.  A  common  type  of  spike-tooth,  iron-framed  harrow.  It  operates 
as  a  shallow  cultivator,  and  may  often  be  very  effective  in  mulching  the  soil 
and  conserving  moisture. 

ture  will  hold  in  the  four  feet  surface  9  acre-inches 
of  water;  clay  soil,  16;  and  a  muck  soil,  40  inches;  which 
figures  are  directly  comparable  for  purposes  of  crop- 
production. 

The  method  used  in  stating  the  moisture  content 
of  a  soil  will  therefore  depend  upon  the  line  of  investiga- 
tion and  the  application  of  the  results  to  be  made.  Both 
the  percentage  of  dry  weight  and  the  percentage  of 
volume  will  be  used  in  this  book,  according  to  the 
point  of  view  of  the  discussion. 


y  FORMS    OF   SOIL   MOISTURE  141 

V 

68.  Forms  and  availability. — There  are  three  forms 
in  which  water  may  exist  in  soils:  (1)  Gravitational 
water,  or  that  which  is  free  to  move  through  the  soil 
under  the  influence  of  gravity.  (2)  Capillary  or  film 
water,  or  that  which  is  held  against  gravity  by  the 
surface  tension  of  the  films  of  water  surrounding  the 
soil  particles.  (3)  Hygroscopic  moisture,  or  that  which 
condenses  from  the  atmosphere  on  the  surface  of  the 

FORMS  OF  SOIL  WATER 

HYGROSCOPIC  CAPILLARY  GRAVITATIONAL 


UNAVAILABLE  AVAILABLE  INJURIOUS 

AVAILABILITY  OF  SOIL  WATER  TO  PLANTS 

Fio.  43.    Diagram  illustrating  the  forms,  proportions  and 

availibility  of  soil   water. 

soil  particles,  when  the  soil  is  allowed  to  become  air 
dry. 

There  is  no  sharp  change  in  the  moisture  condition 
of  the  soil  in  passing  from  one  form  to  the  other.  Still, 
it  is  true  that  there  are  certain  marked  changes  in  some 
of  the  physical  properties  of  the  soil,  such  as  volume, 
weight  and  resistance  to  penetration,  which  are  in  a 
general  way  associated  with  these  transition  points. 

Not  all  of  the  water  in  the  soil  is  available  to  use  of 
plants.  It  is  a  matter  of  general  experience  that  for 
most  farm  crops  the  saturated  condition  of  the  soil 
is  unfavorable  to  the  best  development.  There  are, 
of  course,  many  plants  which  are  adapted  to  such  con- 


142         THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

ditions,  as  for  example  the  swamp  type  of  vegetation. 
About  the  only  cultivated  crops  of  this  sort  are  rice 
and  cranberries.  Practically  all  of  the  common  culti- 
vated crops,  from  vegetables  to  fruit  trees,  are  adapted 
to  growing  in  soil  from  which  the  gravitational  moisture 
has  been  removed.  The  gravitational  water  is  directly 
injurious  to  the  growth  of  these  plants,  and  its  practical 
removal  from  the  soil  constitutes  the  practice  of  agri- 
cultural drainage,  later  to  be  considered  as  a  phase 
of  soil  management.  It  may  therefore  be  stated  that 
gravitational  water  in  the  root  zone  is  injurious  to  most 
farm  crops,  and  consequently  it  is  in  a  sense  unavailable. 
It  is  the  film  or  capillary  moisture  which  supports  plants. 
The  roots  of  ordinary  crops  are  adapted  to  take  the 
moisture  needed  by  threading  their  way  between  the 
soil  particles,  where  they  may  come  in  intimate  contact 
with  these  moisture  films  and  absorb  the  needed  supply 
of  water,  without  being  excluded  from  the  air  supply 
which  promotes  their  growth.  For,  in  the  capillarily 
moist  soil,  the  water  is  retained  chiefly  in  the  very 
small  spaces,  and  the  large  spaces  are  occupied  by  air. 
While  capillary  moisture  is  practically  the  only  form 
upon  which  plants  depend,  it  is  not  possible  for  them 
to  use  all  of  this  form  of  moisture  in  the  soil.  They  take 
their  supply  most  readily  when  the  films  are  relatively 
thick,  and  when  the  globules  between '  the  particles 
are  large.  But,  as  the  thickness  of  the  films  is  reduced 
by  the  use  of  the  plant  and  by  evaporation,  it  becomes 
increasingly  difficult  for  the  plant  roots  to  take  their 
needed  supply.  Before  all  of  the  capillary  moisture  has 
been  removed,  this  difficulty  becomes  so  great  that  it 


HYGROSCOPIC   MOISTURE   IN   SOIL  143 

practically  amounts  to  the  prohibition  of  further  extrac- 
tion by  the  plant.  At  this  stage,  if  evaporation  from 
the  leaves  continues,  the  plants  wilt,  because  they  are 
not  supplied  with  moisture  by  the  roots  as  rapidly  as 
it  is  being  lost. 

Since  plants  cannot  utilize  all  of  the  capillary  moisture 
it  is  manifestly  impossible  for  them  to  derive  any  benefit 
from  the  hygroscopic  moisture,  which  is  held  much 
more  intimately  by  the  soil  particles  than  is  the  capil- 
lary moisture.  In  other  words,  the  hygroscopic  moisture 
capacity  of  a  soil  represents  that  much  water  unavail- 
able to  plants,  to  which  must  be  added  the  proportion 
of  the  capillary  moisture  which  is  also  unavailable. 

69.  Amounts  of   each    form. — The   relative   amount 
of  each  form  of  water  varies  with  the  soil,  and  is  deter- 
mined by  its  physical  properties.    The  forms  of  water 
merge  one  into  the  other. 

70.  Hygroscopic    water. — The   amount    of    each    of 
the  three  forms  of  soil  water  depends  on  the  physical 
properties  of  the  soil.    These  are  best  explained  by  first 
considering    the    hygroscopic    capacity.     This    depends 
on  the  texture  of  the  particles  and  the  content  of  organic- 
matter.     Since   hygroscopic   moisture   is   a   function   of 
the  surface  exposed,  it  results  that  the  larger  the  surface 
area  exposed  by  the  soil  particles,  the  greater  the  hygro- 
scopic   capacity    of   the   soil.     Reference    to    the    table 
on  page  8.3  shows  fine-textured  or  clay  soils  to  have  the 
greatest  surface  area,  and  these  hold  the  most  hygro- 
scopic   moisture.     Sand   soils,    with    a   relatively    small 
surface  area,  hold  a  small  amount  of  this  form  of  water. 
This  fact  is  illustrated  by  the  following  table. 


144         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

Per  cent  of  hygroscopic 
water  at  21°  C. 

Very  fine  sand 1 .8 

Silt 7.3 

Clay 16.5 

Muck 48.0 

The  above  soils  were  pure  separates  derived  by 
mechanical  analysis.  These  figures  serve  to  show  the 
direct  relation  between  the  (1)  surface  area  exhibited 
by  soil  particles  and  the  hygroscopic  moisture  retained. 
The  hygroscopic  moisture  content  of  a  soil  depends 
also  on  the  (2)  temperature,  and  the  (3)  humidity 
of  the  atmosphere.  The  hygroscopic  moisture  decreases 
with  increase  in  temperature.  It  varies  directly  as 
the  relative  humidity  of  the  atmosphere  with  which 
the  soil  is  in  contact.  Consequently,  in  the  air-dried 
condition,  while  a  soil  always  retains  some  moisture, 
it  seldom  exhibits  its  maximum  hygroscopic  capacity. 
Under  average  conditions  of  humidity,  a  light  sand 
may  retain  from  .5  to  1  per  cent,  a  silt  loam  from  2 
to  4  per  cent  and  a  clay  from  8  to  12  per  cent.  This 
is,  of  course,  unavailable  for  the  use  of  plants. 

71.  Capillary  water. — The   capillary  water  capacity 
is    much    larger   than   the   hygroscopic    capacity.     Its 
amount    is  determined   by  three   things:    (1)  Texture, 
(2)  structure;  (3)  content  of  organic  matter. 

72.  Texture. — Texture  is  well  known  to  be  the  great- 
est determining  factor  in   the   water-holding   capacity 
of   soils,    due   to   its   control   of    the   internal   surface, 
and    this   is   particularly  true    with    reference    to    the 
capillary    form.     The    following    table    illustrates    this 
effect  of  texture. 


MOISTURE   IN   SOIL 
TABLE  XIX 


145 


Class 

Per  cent  of  clay 

Per  cent  of 
moisture  retained 
against  force 
2,940  times  that 
of  gravity 

1.  Coarse  sand    .  . 

4.8 

4.6 

2.  Medium  sandy  loam. 

7.3 

7.0 

3    Fine  sandv  loam 

126 

11  8 

4.  Silt  ".  

10.6 

12.9 

5.  Silt  loam  

17.7 

26.9 

6.  Clay  loam  

26.6 

32.4 

7.  Clay     .            

59.8 

46.5 

RELATION  TEXTURE  TO  CAPILLARY 


WATER  CAPACITY 


> 


g20 


A 


7/ 
/* 


TV 


Hut  GRAV 
COARSC  *> 

MEDIUM  s« 


Ht  SAND 

SOIL  SEPARATES 


Fio.  44.    Showing  the  mechanical  composition  of  the  soils  whose  relative 
capillary  water  capacity  is  given  in  Table  XIX. 


No.  1.  Coarse  Sand. 
No.  2.  Medium  Sandy  Loam. 
No.  3.  Fine  Sandy  Loam. 
No.  4.  Silt. 


No.  a.  Silt  Loam. 
No.  6.  Clay  Loam. 
No.  7.  Clay 


146         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

It  must  be  remembered  that  the  hygroscopic  ca- 
pacity of  these  soils  also  increases  with  their  fineness, 
and  that  the  strictly  capillary  moisture  is  represented 
by  the  difference  between  the  total  moisture  content 
given  above  and  the  hygroscopic  moisture. 

The  above  figures  are  the  most  exact  available 
which  show  the  influence  of  texture  upon  moisture 
retention.  But,  while  they  show  the  relative  effect  of 
texture,  they  do  not  indicate  the  amount  of  water 
retained  by  field  soils;  because  these  samples  have 
been  subject  to  a  force  almost  3,000  times  that  of 
gravity,  v/hca  under  the  influence  of  gravity  alone, 
tL"»se  same  soils  will  retain  much  more  water  than  is 
indicated  by  the  figures.  However,  this  influence  of 
gravity  introduces  a  modification  in  the  moisture  content 
of  the  soil  which  must  be  constantly  kept  in  mind. 
Moisture  is  retained  in  the  soil  as  a  result  of  two  sets 
of  forces.  These  are,  first,  the  attraction  of  the  soil 
for  water,  or  adhesion.  For  example,  if  a  marble  is 
ed  into  water  and  withdrawn,  it  carries  with  it 
.n  of  water  over  its  entire  surface.  This  shows  that 
certain  small  distance  from  its  surface,  the  marble 
CA  a  stronger  pull  on  the  water  than  the  water 
exeri,-  for  itself.  If  the  marble  were  dipped  into  mercury 
instead  of  water,  it  would  come  out  with  a  dry  surface, 
because  in  this  case  the  attraction  of  the  mercury  for  its 
own  substance  is  greater  than  the  attraction  of  the 
marble  for  the  mercury.  Quinke  estimates  the  appreci- 
able range  of  this  attraction  to  be  approximately  .002 
millimeter,  which,  it  will  be  remembered,  is  equivalent 
to  the  diameter  of  a  medium-sized  clay  particle.  Its 


RETENTION   OF   SOIL   MOISTURE 


147 


tendency  is  to  arrange  upon  the  surface  of  the  soil 
particles  a  film  of  water  molecules  equivalent  to  this 
thickness. 

But,  because  of  the  second  set  of  forces  the  film  is 
always  thicker  than  this  range  of  molecular  attraction 
of  the  solid.  This  is  due  to  the  attrac- 
tion of  the  water  particles  for  each 
other,  or  cohesion.  The  water  mole- 
cules hang  together.  This  cohesion  of 
the  water  molecules  is  exhibited  in 
surface  tension  which  will  permit  a 
clean  steel  needle  to  be  suspended 
upon  the  surface  of  water,  or  makes 
possible  the  common  trick  of  putting 
a  handful  of  nails  into  a  goblet  already 
level  full  of  water.  This  surface  ten- 
sion acts  like  a  stretched  elastic  mem- 
brane, and  permits  the  water  to  be 
piled  up.  This  is  what  happens  in 
the  soil  when  capillarity  comes  into 
play.  As  a  result  of  these  two  sets 
of  attraction,  the  water  hangs  on  the 
particles  in  thick  films;  and  it  drops 
away  only  when  the  weight  of  the 


Fio.  45.  f 
distributior     , 
on   column.- 
cal    parti  ok  _ 
ent  texture 
accumulatio 
in  the  lowe 


1    -    +  Vi 

& 

the  soil 

water  becomes  greater  than  the  surface  tension'?"" 
liquid. 

It  is  clear  that  soil  forms  a  column  of  considerab. 
height,  and  further,  that  the  closer  the  water  film  is 
drawn  around  the  soil  particles,  the  thinner  it  will  be, 
and  consequently   the  less   water  it   will   contain.     To 
illustrate:  Suppose  a  cylinder  to  have  flexible  rubber 


148 


THE  PRINCIPLES   OF   SOIL  MANAGEMENT 


diaphragms  stretched  across  at  frequent  intervals  from 
the  top  to  the  bottom.  If  now  a  heavy  ball  is  dropped 
upon  the  upper  membrane,  it  will  be  weighed  down 
upon  the  next  membrane  below,  and  this  in  turn  will 
be  depressed,  until  the  ball  has  brought  enough  of  the 
membranes  in  contact  to  support  its  weight.  Under  these 
conditions,  the  upper  membrane  will  be  stretched  most 
severely,  and  will  therefore  be  thin,  while  the  lower 


6  10  16  20  26  30  86  1ft 

PERCENT.  OF  WATER 

Z   Qf  6-    Curves  showing  the  distribution  of  water  in  columns  of  soil 
'•apillarily  saturated,  as  given  in  Table  XX. 

cerl 

ne   will   be   very   slightly   stretched.     If,    now, 

"A.  *  I 

exei  foi^ate  tne  actual  amount  of  rubber  in  each  section 
instead  e*-^11^61"  ^  w*^  ^e  ^oun^  smallest  at  the  top 
kecauf<argest  at  the  bottom. 

owp  n  the  same  way,  gravity  affects  the  distribution 
r»/i  water  in  the  soil.  It  forms  thick,  bulging  films  in 
the  lower  part  of  the  column,  and  thin,  closely  drawn 
films  at  the  top  of  the  column.  Consequently,  the  sur- 
face of  a  soil  of  uniform  texture  is  normally  less  moist 
than  the  subsoil. 


DISTRIBUTION  OF  SOIL  MOISTURE 


149 


As  a  result  of  this  fact,  it  is  not  practicable  to  say 
that  any  soil  contains  a  definite  uniform  per  cent  of 
capillary  moisture.  The  content  varies  with  the  height 
of  the  column  and  the  plane  in  the  column  at  which 
the  determination  may  be  made.  This  important 
principle  in  the  distribution  and  amount  of  moisture 
in  the  soil  is  well  illustrated  by  the  following  tables 
and  curves,  for  soils  of  different  texture,  as  obtained 
by  Buckingham: 

TABLE  XX 


Per  cent  of  water  at  different  distances  from 
bottom  of  column  in  inches 

2 

10 

20 

30 

40           50 

1  Clean  dune  sand 

27.0 
23.0 
28.5 
29.0 
35.0 

64.0 

23 
14 
25 
23 
23 

55 

7 
10 
16 
21 

18 

47 

3.5 
7.5 
9.5 
19.0 
15.0 

36.0 

3 
5 

7 
17 
11 

20 

2.  Coarse  sand  
3.  Fine  sandy  loam  
4.  Light  silt  loam  

5.  Clay  

6.  Heavy  loam,   rich   in 
humus 

The  above  moisture  curves  illustrate  very  clearly  the 
accumulation  of  the  water  in  the  lower  part  of  the  soil 
column.  These  columns  were  permitted  to  stand  in 
contact  with  water  for  many  days,  so  that,  with  the 
possible  exception  of  the  finest  textured  soils,  they  had 
come  to  equilibrium.  It  will  be  noted  that  the  difference 
in  moisture  content  is  much  greater  at  the  top  of  the 
columns  than  at  the  bottom,  and  decidedly  greater 
than  at  a  height  of  about  ten  inches  above  the  water. 


150 


THE   PRINCIPLES    OF  SOIL   MANAGEMENT 


When  two  soils  of  different  texture  are  placed  in 
contact  with  moisture  free  to  move  from  one  to  the 
other,  they  come  into  moisture  equilibrium  after  a  time, 
and  each  holds  a  certain  proportion  of  the  water.  The 
curvature  of  the  water  surfaces  between  the  particles 

80T 


NO. 2  FINE  SAND 

VERY  FINE  SAND 


NO. 4  CLAY 


NO.1  FINE  GRAVEL 
COARSE  SAND 
MEDIUM  SAND 

SOIL  SEPARATES 

Fio.  47.    Curves  showing  the  mechanical  composition  of  the  soils  whose 
capillary  moisture  capacity  is  shown  in  Table  XX  and  Fig.  46. 

of  the  two  soils  is  the  same.  But  since  in  a  given  volume 
of  soil  the  fine  texture  has  so  many  more  of  these  indi- 
vidual drops  of  water,  its  total  content  is  greater  than 
that  of  the  coarse-textured  soil.  This  matter  of  the 
curvature  of  the  water  surfaces  in  the  soil  will  come  up 


STRUCTURE  AND  SOIL   MOISTURE  151 

prominently,  in  considering  the  capillary  movement 
of  moisture.  The  relative  adjustment  and  distribution 
of  the  moisture  between  small  and  large  particles  in 
contact  is  illustrated  in  Fig.  45.  When  in  capillary 
equilibrium,  two  soils  should  appear  equally  moist. 

73.  Structure. — Structure  is  the  second  factor  which 
determines  the  moisture  capacity  of  a  soil.  If  the  state- 
ments in  reference  to  the  effect  of  texture  have  been 
fully  understood,  the  influence  of  structure  will  be 
readily  grasped.  The  effect  of  structure  is  to  alter  the 
effective  size  of  the  soil  units  or  granules,  and  also  of 
the  spaces  which  they  form.  In  a  coarse  sand  soil, 
the  general  effect  of  rendering  the  structure  of  the  soil 
more  loose  is  to  proportionately  reduce  its  water-holding 
capacity,  because  the  spaces  are  already  so  large  as  to 
hold  a  relatively  small  amount  of  water,  and  that  to 
a  very  limited  height.  Change  in  structure  further  de- 
creases that  already  deficient  capacity.  On  the  other 
hand,  in  a  fine  clay  soil  the  spaces  are  all  very  small, 
and  all  have  a  capillary  efficiency  to  a  great  height.  This 
height  is  much  more  than  is  ordinarily  needed  to  bring 
the  moisture  from  the  deep  subsoil  to  the  root  zone. 
In  such  a  soil  a  more  loose  and  open  structure  has  the 
effect  of  increasing  the  effective  moisture  capacity, 
so  long  as  the  spaces  are  still  able  to  hold  water  at  the 
surface  of  the  column.  But  when  this  maximum  size 
of  space  is  exceeded,  as  in  a  coarsely  clodded  soil,  the 
moisture  capacity  drops  low,  as  in  the  case  of  sand  or 
gravel,  when  growth  may  lie  seriously  interrupted. 
Ordinarily,  then,  it  may  be  said,  that  loosening  the 
structure  of  a  coarse  sand  or  gravel  soil  lowers  its 


152 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


moisture-holding  capacity  while  a  reasonable  granulation 
of  a  clay  soil  increases  its  moisture-retaining  capacity. 

This  effect  of  structure  on  the  moisture  capacity 
of  two  soils  is  illustrated  by  the  following  curves,  based 
upon  the  results  of  Buckingham.  The  mechanical 
analysis  of  these  soils  may  be  found  in  curves  already 
given.  (See  page  150.)  The  sandy  loam  is  No.  3,  and  the 
clay  is  No.  5. 


0 

0  10  20  30  40  50 

FIG.  48.  Curves  showing  the  distribution  of  water  in  columns  of  sand  and 
clay  when  loose  and  compact  and  capillarily  saturated.  Figures  given  in 
Table  XXI. 

TABLE  XXI. — PER  CENT  OP  WATER  IN  SAND  AND  CLAY,  LOOSE 
AND  COMPACT 


Soil 

Structure 

Dry 

porosity 
per  cent 

Per  cent  of  moisture  at  different  heights 
above  water  level 

2  in. 

10  in. 

20  in. 

30  in. 

40  in. 

Sandy  loam.  . 
Clay  

Loose  

50 
35 
59 
52 

28.0 
27.0 
42.5 
34.0 

25 
25 
32 
23 

16.0 
17.5 
28.0 
18.0 

9.0 
12.5 
27.0 
15.0 

6 
10 
26 
12 

Compact    .  . 
Loose  

Compact   .  . 

ORGANIC  MATTER  AND  SOIL  MOISTURE  153 

74.  Content  of  organic  matter. —  Organic  matter, 
especially  in  the  form  of  humus,  has  a  larger  capacity 
for  moisture  than  has  the  mineral  portion  of  the  soil. 
Aside  from  the  fact  that  such  material  has  a  large  inher- 
ent moisture  capacity,  and  that  in  proportion  to  its 
amount  in  the  soil  it  increases  the  water  capacity, 
no  exact  figures  can  be  given.  The  moisture  content 
of  such  material  varies  with  the  stage  of  decay,  as  well 
as  the  general  physical  properties  of  the  material.  The 
following  figures  compiled  by  Storer  illustrate  this 
capacity. 

Per  cent  of 
water  retained 

1.  Humic  acid  extract  from  peat 1,200 

2.  Non-acid  humus  prepared  from  peat 645 

3.  Ordinary  vegetable  mold 190 

4.  Peat 201-309 

5.  Garden  loam,  54  per  cent  clay,  7  per  cent  humus.  96 

6.  Dark  Illinois  prairie  soil 57 

7.  Mucky  soil  (weighing  30  pounds  per  cubic  foot) .  75 

Besides  its  inherent  capacity,  organic  matter  affects 
the  moisture  capacity  through  its  influence  on  soil 
structure.  In  clay  it  produces  a  desirable  condition 
of  granulation  and  therefore  increases  the  absolute 
moisture  capacity.  And  its  addition  to  sand  has  a  similar, 
though  smaller  effect.  This  is  illustrated  by  the  follow- 
ing figures,  obtained  by  Detmer,  as  quoted  by  Storer, 
which  resulted  from  the  mixture  of  sand  and  muck. 
It  will  be  noted  that  in  proportion  as  muck  is  substi- 
tuted for  an  equal  weight  of  sand,  the  water  capacity 
of  the  mixture  is  increased,  as  is  well  shown  by  the 
ratio  in  the  last  column. 


154         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 
TABLE  XXII 


Per  cent  of  sand 

Per  cent  of  muck 

Grams  of  water 
absorbed 

Ratio  of  absorption 
of  water  in  sand 
and  in  mixture 

100 

12.2 

1  :1 

80 

20 

24.0 

1  :2 

60 

40 

42.0 

1  :3.5 

40 

60 

71.7 

1  :6.0 

20 

80 

99.1 

1  :8.0 

100 

114.4 

1  :9.3 

76.  Volume  of  water  held  by  different  soils. — The 
columns  of  soil  from  which  the  figures  presented  on 
page  148,  with  the  accompanying  curves,  were  obtained, 
were  forty-five  inches  in  height,  with  their  lower  ends 
dipping  in  water.  As  they  were  run  for  several  months, 
their  moisture  content  represents  the  maximum  capacity 
for  each  soil.  Under  these  conditions,  the  mean  moisture 
content  was  as  follows: 

TABLE  XXIII 


I 

II 

III 

IV 

Dry 

Approximate 

poros- 

Final mean 

per  cent  of 

Per  cent  of 

ity 

water  content 

moisture  at 

available 

Per 

Per  cent 

which  crops 

moisture 

cent 

will  wilt 

1.  Dune  sand  

52 

10.7 

3 

7.7 

2.  Coarse  sand  

51 

10.6 

3 

7.6 

3.  Fine  sandy  loam  . 

50 

18.0 

5 

13.0 

4.  Light  silt  loam  .  . 

50 

20.9 

10 

10.9 

5.  Clay    

59 

30.4 

17 

13.4 

6.  Muck  soil   

80* 

250.0 

80 

170.0 

•Estimated. 


WATER   CAPACITY   OF  SOILS 
TABLE  XXIII,  continued 


155 


V 

VI 

VII 

Weight  of 

Volume  of  available 

Inches  of  avail 

dry  soil  per 

water  per 

able  water 

cubic  foot 

cubic  foot 

to  depth  of 

four  feet 

Lbs. 

cu.  in. 

c.c. 

1.  Dune  sand  .... 

80 

166 

2,720 

4.60 

2    Coarse  sand 

81 

170 

2,790 

5.20 

3.  Fine  sandy  loam  . 

83 

300 

4,900 

8.50 

4.  Light  silt  loam  .  . 

83 

250 

4,100 

6.90 

5.  Clay  

68 

252 

4,140 

7.03 

6.  Muck  soil  

15 

740 

11,550 

20.50 

But  all  of  this  moisture  is  not  available  to  crops. 
The  third  column  gives  the  per  cent  of  water  in  these 
soils  which  would  be  unavailable,  or  the  point  at  which 
plants  would  ordinarily  wilt.  This  per  cent,  or  amount 
of  water  at  which  plants  are  just  able  to  survive,  is 
termed  the  minimum  or  critical  moisture  content,  while 
the  highest  per  cent  at  which  the  plant  will  survive 
is  termed  the  maximum  moisture  content.  The  inter- 
mediate point  at  which  any  crop  makes  its  best  growth 
is  termed  the  optimum  moisture  content. 

Each  of  these  points,  or  moisture  conditions,  is  very  • 
definite  for  each  soil  and  for  each  crop.  The  minimum 
for  different  crops  on  the  same  soil  is  not  the  same  as 
the  results  of  a  number  of  investigators  have  shown. 
Storer  reports  that,  on  a  calcareous  soil  having  a  hygro- 
scopic capacity  of  5.2  per  cent,  the  minimum  for  grasses 
was  9.85  per  cent,  and  for  legumes  10.95  per  cent;  while, 
on  peat  (muck)  with  a  hygroscopicity  of  42.3  per  cent, 
the  grasses  suffered  at  50.87  per  cent  of  moisture, 


156 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


legumes  at  52.87  per  cent  of  moisture.  Warington 
concludes,  from  the  results  of  Hellriegel  and  Wollny, 
that  "when  the  soil  contains  80  per  cent  of  the  water 
required  to  saturate  it,  the  proportion  was  too  high; 
and  that  when  the  water  amounted  to  only  30  per  cent 
of  saturation,  the  proportion  was  too  low  for  the  pro- 
duction of  a  maximum  crop.  The  largest  crops  were 
obtained  when  the  proportion  of  water  lay  between 
40  and  60  per  cent  of  that  required  for  full  saturation." 


ill     a     4     0     <     7     8     »    10   11   12  13  14  16  16   17  UllS  20  21  22   23  24  2S   2«  27  J8   »  30 


B    ••    M  o  W  •  9  ft  «t 

-i    =>    d  d          =*     RAINFALL   IN   INCHES        3  ef          d 

FIG.  49.    Curve  showing  moisture  content  of  a  light  sandy  loam — early- 
truck  soil,  Union  Springs,  Alabama,  June,  1896. 

Cameron  and  Gallagher  have  shown  that  the  maxi- 
mum and  minimum  points  are  marked  by  distinct 
changes  in:  (1)  The  cohesion  of  the  soil.  (2)  Its  volume 
weight.  (3)  The  freedom  with  which  the  soil  gives  up 
moisture.  The  first  of  these  facts  is  of  especial  import- 
ance in  the  tillage  of  soil.  Between  the  maximum  and 
the  minimum  points  the  soil  "works"  at  its  best.  It 
does  not  puddle,  and  it  is  sufficiently  moist  to  give  that 
desirable  state  of  granulation  which  is  expressed  by 
good  tilth.  The  clods  of  the  clay  soil  are  not  hard,  and 


WATER   CONTENT   OF  FIELD  SOILS 


157 


therefore  pulverizing  operations  attain  their  maximum 
efficiency  with  the  minimum  of  work.  (See  page  103.) 
A  soil  always  tilled  in  this  condition  should  never  get 
into  bad  tilth. 


2   18 


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\ 

^ 

s 

•^~, 

/ 

^~- 

"—  - 

*»0 

sr 

1  — 

^ 

r 

~C 

0* 

ff 

-  —  ' 

— 

/ 

—  " 

5 

/ 

•^ 

IN 

:  c 

F 

DF,C 

Ul 

H 

.01 

RAINFALL  IN  INCHES 

Fio.  50.    Curve  showing  moisture  content  of  silt  loam — blue-grass  soil, 
Lexington,  Kentucky,  September,  1896. 

76.  Available  water  in  some  field  soils. — The  actual 
moisture  content  and  fluctuations  through  a  part  of  the 
growing  season  for  different  soils  is  always  of  prime 
concern  to  the  farmer.  The  following  curves,  (Figs. 


i  SO  81  *!  «3  S4  Si  *«  *7  28  M  3U  81 


Fio.  51.    Curve  showing  moisture  content  of  clay  soil — black  cretaceous 
prairie — Macon,  Mississippi,  July,  1896. 

49,  50  and  51)  based  upon  the  results  of  Whitney  and 
Hosmer,  illustrate  these  fluctuations. 

If  we  assume  for  the  above  soils  a  porosity  of  47,  52, 
and  65  per  cent,  respectively,  their  weights  per  cubic 


158 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


foot  would  be  88,  79  and  58  pounds  each.  Using  the 
maximum  and  minimum  moisture  contents  indicated 
by  the  above  curves,  the  available  moisture  retained 
by  each  of  the  soils  is  as  follows: 

TABLE  XXIV 


Water  capacity 

Amount  of  available  water 

Minimum 
Per  cent 

Maximum 
Per  cent 

Per  cent 

Cu.  in.  per 
cu.  ft. 

In.  per 
acre,  4  ft. 

Light      sandy 
loam  
Silt  loam  

3 
15 
23 

8 
25 

40* 

5 
10 
17 

122 
218 
274 

3.4 
6.0 
7.6 

Clay.  . 

It  is  possible  that  the  maximum  assumed  for  the-clay 
is  too  high,  in  which  event  the  available  moisture  in 
the  fifth  column  for  this  soil  is  also  too  high;  but  field 
experience  indicates  that  it  is  reasonable. 

By  reference  to  page  134,  giving  the  amount  of  water 
required  to  produce  a  crop,  it  will  be  observed  that  the 
surface  four  feet  of  the  sand  soil  will  not  retain  enough 
water  for  the  medium  crop  yield,  and  that  the  clay 
soil  contains  less  than  half  enough  water  for  a  large 
yield  of  many  crops  under  the  best  management.  This 
necessitates  the  replenishment  of  the  supply  by  rainfall, 
irrigation,  or  movement  up  .from  the  subsoil,  after  the 
best  tillage  practice  has  been  employed  to  prevent 
unnecessary  loss  by  evaporation. 

77.  Relation  of  surface  tension  to  capillarity. — In 
addition  to  the  three  factors  mentioned  as  controlling 

*  Assumed. 


SURFACE   TENSION  AND   CAPILLARITY  159 

the  capillary  moisture  capacity  of  a  soil,  one  other  is  to 
be  considered.  The  surface  tension,  or  cohesiveness, 
of  the  moisture  was  described  as  one  of  the  forces  which 
acts  in  conjunction  with  the  texture,  structure  and  or- 
ganic content,  to  retain  water.  The  surface  tension 
of  any  liquid  is  not  a  constant  quantity,  and  the  soil 
water  is  no  exception  to  this  rule.  Anything  which  in- 
creases surface  tension  increases  moisture  retention, 
and  likewise  anything  which  decreases  surface  tension 
decreases  the  moisture  retention.  Soil  moisture  is  sub- 
ject to  considerable  variation  in  surface  tension.  Two 
things  are  most  active  to  change  this  tension,  or  co- 
hesiveness. These  are:  (1)  Materials  in  solution  in 
the  water.  Lime  and  many  other  salts  increase  the 
tension,  some  substances  decrease  it  below  the  normal 
for  pure  water.  (2)  Changes  in  temperature  alter  the 
surface  tension. 

Whitney  and  others  have  determined  the  surface 
tension  of  a  number  of  salt  and  soil  solutions,  some 
of  which  are  given  in  the  table  on  the  following  page. 
The  concentrations  are  not  uniform. 

The  figures  show  that  many  salts  increase  the  surface 
tension  of  the  soil  moisture  above  that  for  pure  water, 
and  that  certain  other  substances  decrease  the  surface 
tension.  Among  the  latter  are  some  of  the  most  common 
constituents  of  manures  which  greatly  decrease  surface 
tension.  All  oily  or  fatty  substances  reduce  the  tension, 
and,  since  both  these  latter  are  present  in  nearly  all 
soils,  the  average  surface  tension  of  the  soil  moisture 
is  less  than  that  of  pure  water.  Consequently,  the 
tendency  is  to  retain  less  of  such  a  solution  than  of  pure 


160 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


TABLE  XXV 


Solution 


Specific 
gravity 


Surface  tension 
dynes  per  sq.  cm. 


Water 1.0000 

Common  salt  (NaCl) 1.1000 

Muriate  potash  (KC1) 1.1000 

Ammonium  sulfate  ( (NHJ2  SOJ .  1 . 1000 

Sodium  sulfate  (Na3SO4) 1.1000 

Sodium  nitrate  (Na  NO3) 1.1000 

Potassium  hydrate  (KOH) 1.1000 

Potassium  sulfate  (K2SO4) 1.0830 

Wood  ashes 1.0038 

Thomas  slag 1.0012 

Marl 1.0013 

Lime 1.0020 

Ammonia  (NH4OH)    0.9600 

Urine 1.0260 

Stable  manure 1.0013 

Kentucky  Blue  Grass  soil 1.0000 

Wheat  soil 1.0000 

Garden  soil..  1.0000 


73.9 
77.6 
77.5 
76.8 
75.8 
75.8 
75.1 
75.1 
75.2 
77.4 
77.0 
75.5 
67.5 
64.9 
73.2 
71.0 
69.6 
69.4 


water.  Various  salts  in  solution  as  fertilizers  or  otherwise, 
tend  to  overcome  this  weakness,  and  therefore  to  in- 
crease the  moisture  capacity. 

Increase  in  temperature  decreases  the  surface  ten- 
sion, until  near  the  boiling  point  it  is  almost  nil.  Briggs 
reports  that  at  O°C.  the  tension  of  pure  water  is  75.6 
dynes  per  square  centimeter,  and  at  25°C.  it  is  72.1. 
At  the  lower  temperature  more  water  is  held  in  the 
soil,  and  this  is  one  reason  why  soils  appear  more  moist 
in  cool  seasons.  (See  also  page  183.) 

78.  Gravitational  water. — By  reference  to  the  ori- 
ginal illustration  (page  141),  it  will  be  noted  that  the 
gravitational  water  was  denned  as  that  portion  in  excess 


GRAVITATIONAL   WATER 


161 


of  the  hygroscopic  and  capillary  capacity  of  a  soil. 
It  is  not  retained  by  the  same  forces,  and  is,  therefore, 
free  to  move  under  the  influence  of  gravity,  in  so  far 
as  the  condition  and  character  of  the  soil  will  permit. 
The  amount  of  gravitational  water  depends  on  the  total 
pore  space  of  the  soil  on  the  one  hand,  and  on  the  total 
hygroscopic  and  capillary  capacities  on  the  other  hand. 
It  is  the  difference  between  the  total  capacity  of  the 
soil  for  water  and  that  held  in  the  other  two  forms. 
It  is  measured  by  that  amount  which  will  flow  from  a 
soil  having  all  of  its  pores  filled  with  water. 

The  maximum  water  capacity  of  a  soil  refers  to  the 
total  amount  of  water  which  can  be  put  in  a  given 
volume  of  soil.  It  is  therefore  determined  directly 
by  the  total  pore  space  of  the  soil.  The  pore  space  may 
range  from  35  per  cent  in  a  clean  sand  to  60  or  70  per 
cent  in  a  well-granulated  clay,  and  to  SO  or  90  per  cent 
in  a  muck  soil.  If  we  assume  the  weight  per  cubic  foot 
for  these  materials  given  on  page  155,  the  maximum 
per  cent  of  water  is  as  follows: 

TABLE  XXVI 


I 

Weight 
per  cu.  ft. 
Pounds 

II 

Per  cent 
pore  space 

III 

Pounds  of 
water 
per  cvi.  ft. 

IV 

Per  rent 
of  wuipr 
in  soil  at 
saturation 

1.  Dune  sand.  .  .                          80 

52 

32  .") 

40.5 

2.  Coarse  sand  81 
3.   Fine  sandy  loam  .                   83 

51 
50 

32.0 
31.5 

39.5 
3S  0 

4.   Light  silt  loam                         83 

50 

31.5 

3SO 

5.  Clay      .                                    68 

59 

37  0 

54  5 

6.  Humus.  .  .                                  15 

80 

500 

333  0 

162 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


One  effect  of  moisture  on  porosity  is  to  be  noted 
here.  When  a  dry  soil  imbibes  water  it  expands,  so  that, 
when  the  porosity  is  determined  in  a  wet  soil,  it  is  always 
found  to  be  larger  than  in  the  same  soil  when  dry. 
This  expansion  is  greatest  in  the  fine-textured  soil, 
and  in  muck  it  is  at  the  maximum.  There  are  several 
factors  which  enter  into  this  result,  one  of  which  is 
the  tendency  of  the  soil  moisture  to  float  the  particles, 
so  that  they  rest  together  with  less  force  than  when 
the  soil  is  dry.  Gallagher  has  shown  that  a  sample  of 
muck  soil  having  a  hygroscopic  capacity  of  above  40 
per  cent  lost  29.2  per  cent  of  its  original  volume  in  dry- 
ing from  a  moisture  content  of  210  per  cent  to  one 
of  about  80  per  cent. 

In  Table  XXIII  on  page  154  is  given  the  maximum 
capacity  and  approximate  wilting  point  of  the  soils 

TABLE  XXVII 


I 

II 

III 

IV 

V 

VI 

V 

2-3 

«* 

S 

"3 

o 
~5  ~^ 

,  o 

|| 

*1 

§9 

(ft. 

S's 

B  „ 
•  .0  C 

S.sg 
•Bfi-« 

t||| 

5  S 

.2 

Hn" 

IJ 
la 

it 

11 

•?-2^- 
a  °^ 

•IE3S 

jj 

«.5 

• 

O 

C5     2 

.3  C*£ 

s 

a 

S. 

- 

Per  cent 

Per  cent 

Per  cent 

1    Dune  sand.  . 

80 

10  7 

40  5 

29  8 

23  8 

1-38 

2.  Coarse  sand  

81 

10.6 

39  5 

289 

234 

1-37 

3.  Fine  sandy  loam  .  . 

83 

18.0 

38.0 

20.0 

16.6 

1  :2.1 

4.  Silt  loam    

83 

20.9 

380 

189 

15  7 

1-18 

5.  Clay    

68 

304 

54  5 

13  9 

9  5 

1-18 

6.  Muck  soil  

15 

250.0 

3330 

830 

12  5 

1-13 

GRAVITATIONAL   WATER   AND   DRAINAGE         163 

recorded  in  the  last  table.  If  this  per  cent  be  subtracted 
from  the  per  cent  given  in  Column  IV  of  the  last  table, 
the  per  cent  of  actual  gravitational  water  in  those  soils 
may  be  determined.  This  is  shown  by  the  preceding 
table. 

The  amount  in  Column  V  represents  the  pounds 
of  water  per  cubic  foot  which  would  be  lost  by  drainage 
from  each  of  the  soils  if  their  pores  were  all  completely 
filled  with  water.  Such  a  soil  is  said  to  be  saturated. 
That  plane  in  the  soil  to  which  level  all  of  the  pores 
are  filled  with  water — saturated — is  known  as  the 
water-table.  This  region  of  saturation  is  sometimes 
known  as  the  "ground  water." 

It  is  possible  to  have  such  a  structure  in  a  fine  clay 
soil  that  all  of  its  spaces  are  practically  filled  with  water 
held  capillarily.  It  will  be  noted  from  the  table  that 
the  proportion  of  the  total  water  capacity  which  is 
permanently  retained  increases  with  the  fineness  of 
the  soil,  and  consequently  with  the  decrease  in  the 
size  of  the  individual  pores,  as  is  shown  in  Column 
VI.  The  clay  in  the  above  tables  appears  to  be  very 
thoroughly  granulated,  which  is  responsible  for  the 
similarity  in  the  ratios  for  the  silt  and  clay. 

Gravitational  water  is  directly  injurious  to  upland 
crops,  but  when  it  exists  at  a  depth  of  from  four  to  six 
feet  below  the  surface,  it  may  serve  as  a  reservoir  from 
which  moisture  is  withdrawn  by  capillarity,  to  offset 
losses  by  evaporation.  Water  may  be  removed  by 
capillarity  from  the  saturated  zone  to  the  point  where 
the  loss  is  taking  place,  and  under  these  conditions  the 
ground  water — which  then  becomes  capillary  water — 


164         THE   PRINCIPLES   OF  SOIL  MANAGEMENT 

is  directly  beneficial,  and  the  process  constitutes  a  form 
of  natural  sub-irrigation. 

The  figures  presented  above  illustrate  the  effect  of 
texture  on  the  total  water  capacity  of  a  soil,  and  upon 
the  proportion  of  gravitational  water.  Anything  which 
increases  the  pore  space  increases  the  total  water  capac- 
ity. When  there  is  not  a  corresponding  increase  in 
the  capillary  capacity,  as  happens  in  a  sandy  soil,  the 
total  amount  of  gravitational  water  is  thereby  increased. 
That  is,  in  such  a  soil,  there  is  a  larger  amount  of  water 
which  may  be  lost  by  percolation.  In  so  far  as  organic 
matter  alters  the  structure  of  the  soil,  it  modifies  the 
gravitational  water  content  of  a  soil  in  the  manner 
just  outlined. 

79.  Amount  and  rate  of  loss. — Near  the  outset  of 
the  discussion  of  soil  moisture,  it  was  stated  that  the 
amount  of  water  in  a  soil  depends  upon  the  extent  and 
rate  of  loss  of  water,  as  well  as  upon  the  factors  which 
have  just  been  explained.  For  example,  fifteen  inches 
of  water  is  far  more  efficient  in  crop  production  when 
applied  to  a  loam  soil  in  a  humid  region,  like  the  New 
England  states,  than  when  applied  to  the  sand  of  the 
Imperial  Desert,  California.  In  the  latter  case,  the  loss 
by  percolation  and  evaporation  is  so  great  and  so  rapid 
that  the  amount  of  moisture  available  to  crops  is  very 
small.  The  two  forms  of  loss  which  affect  the  moisture 
in  the  soil  are:  (1)  Percolation.  (2)  Evaporation. 

Percolation  is  the  gravitational  flow  of  water  through 
the  pores  of  a  soil.  Percolation  concerns  the  gravitational 
water.  The  total  loss  in  any  given  soil  will  depend  upon 
the  distribution  of  the  rainfall  or  the  irrigation  supply. 


MOVEMENT  OF  SOIL   WATER  165 

Evaporation  takes  place  at  the  surface,  and  from 
the  plants  growing  in  the  soil.  The  rate  of  such  loss 
depends  on  the  climatic  conditions.  In  those  regions 
where  the  rainfall  comes  in  frequent  small  showers, 
which  wet  the  soil  to  a  depth  of  only  a  few  inches,  a 
very  large  proportion  of  this  water  is  immediately 
returned  to  the  surface  by  capillarity,  and  lost  by  evapo- 
ration. On  the  other  hand,  if  the  rainfall  occurs  at  long 
intervals  and  in  large  amounts,  so  that  it  percolates 
deeply  into  the  subsoil,  it  may  be  held  there  by  appro- 
priate surface  tillage. 

III.     MOVEMENT    OF    SOIL    WATER 

Soil  moisture  is  subject  to  movement  in  three  ways. 
This  movement  may  be  injurious  if  it  facilitates  the 
loss  of  moisture,  which  should  be  retained  for  the  crop; 
it  may  be  beneficial  when  it  serves  to  replenish  the 
moisture  supply  upon  which  the  plant  is  dependent. 
In  the  discussion  of  the  moisture  content  and  capacity 
of  soils,  it  was  pointed  out  that  no  soil  retains  within 
the  surface  four  feet  enough  water  to  meet  the  needs 
of  a  full-crop  yield  under  average  field  conditions. 
This  indicates  the  necessity  for  the  movement  into  the 
root  zone  of  moisture,  to  take  the  place  of  that  removed 
by  the  plant  and  lost  in  other  ways.  The  movement 
of  moisture  from  adjacent  supply  in  the  soil, — as  the 
deep  subsoil — is  just  as  useful  as  the  direct  addition 
of  water  to  the  soil  by  rainfall.  The  three  types  of 
movement  of  soil  moisture  are  (a)  gravitational,  (6) 
capillary  and  (c)  thermal, 


166 


THE   PRINCIPLES   OF  SOIL  MANAGEMENT 


80.  Gravitational  movement. —  Gravitational  move- 
ment is  the  result  of  the  gravity  pull  upon  the  soil 
water.  The  slower  the  downward  movement  of  water, 
the  longer  the  water  will  be  in  the  root  zone  of  the  crop, 
and  therefore  the  greater  use  will  the  plant  be  able 
to  make  of  that  particular  supply  of  moisture.  This 
gravitational  movement  concerns  primarily  the  gravi- 
tational water,  and  is  not  effective  to  move  either  the 
hygroscopic  or  the  capillary  forms  of  water,  although 
these  are  subject  to  the  same  gravity  pull.  The  reason 
is,  so  far  as  these  forms  of  moisture  are  concerned, 
that  the  gravity  pull  upon  them  is  overbalanced  by 
other  forces.  It  will  be  noticed,  in  fact,  that  gravita- 
tional water  is  denned  as  that  part  of  the  soil  water 
which  is  free  to  move  under  the  influence  of  gravity, 
Such  movement  constitutes  percolation. 

The  rate  of  percolation  depends  upon  two  primary 
conditions.  These  are:  (1)  The  texture  of  the  soil. 
(2)  The  structure  of  the  soil.  The  rate  of  movement 
depends  directly  upon  the  diameter  of  the  individual 
soil  spaces.  The  larger  the  size  of  spaces,  the  more  freely 
will  the  water  descend.  King  has  observed  the  following 
movement  of  water  through  sands  of  different  texture 
in  twenty-four  hours: 

TABLE  XXVIII 


Sands 

Clay  loam 

Black  marsh 

Mean  diameter  in  m.m. 

.50 

.35 

.27 

.25 

. 

Inches 
301 

Inches 
160 

Inches 

73.2 

Inches 

39.7 

Inches 
1.6 

Inches 

0.7 

GRAVITATIONAL  MOVEMENT  167 

The  columns  were  one-tenth  of  a  foot  in  cross-section 
and  fourteen  inches  high,  and  a  head  of  two  inches  of 
water  was  maintained  above  the  top  of  the  soil.  These 
figures  show  very  clearly  the  reduction  in  the  flow  of 
water  as  the  texture  becomes  finer. 

Under  field  conditions,  the  percolation  of,  water 
through  the  soil  is  much  facilitated  by  the  presence  of 
numerous  cracks,  root  passages,  and  worm  and  insect 
burrows,  because  of  their  relatively  large  diameter. 

Several  other  factors  affect  the  percolation  of  water. 
The  entrance  of  rain  or  irrigation  water  into  the  dry  soil 
where  it  is  applied  in  a  sheet  over  the  surface  is  hindered 
by  the  presence  of  the  air  in  the  pores  in  the  soil.  If 
the  subsoil  is  dense,  or  is  filled  with  water,  this  inter- 
mediate band  of  air-filled  soil  serves  to  hold  back  the 
surface  water,  except  as  the  air  may  escape  in  bubbles 
through  the  upper  layer.  For  this  reason,  in  part,  a 
heavy  shower  of  rain  sinks  into  the  soil  to  a  very  small 
depth,  and  is  relatively  ineffective.  Entrance  of  the 
water  may  be  greatly  facilitated  by  a  loose  condition 
of  the  soil,  which  affords  quite  large  as  well  as  small 
spaces.  The  large  spaces  are  less  likely  to  be  entirely 
filled  with  water,  and  hence  afford  means  for  the  escape 
of  air,  while  the  water  passes  in  through  the  smaller 
pores.  There  is  another  hint  here  in  the  conservation 
of  rainfall.  If  the  soil  is  in  a  very  loose  condition  to  a 
depth  of  eight  or  ten  inches,  the  water  will  percolate 
into  this  layer,  and  its  movement  will  be  so  much  re- 
tarded that  a  larger  part  will  find  its  way  into  the  deep 
subsoil  and  be  permanently  retained  than  if  the  surface 
soil  is  uniformly  fine. 


168          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

Changes  of  temperature  affect  the  flow  of  water 
through  soils  in  several  ways.  It  affects  the  gravita- 
tional water  directly  by  changing  its  viscosity.  Warm 
water  is  more  limpid  and  flows  more  freely  than  cold 
water,  just  as  oil  is  thinned  by 'heating.  Consequently 
soils  drain  more  readily  in  summer  than  in  winter.  (See 
also  page  183.) 

Changes  in  temperature  also  affect  percolation 
indirectly  through  their  effect  on  the  free  air  in  the  soil, 
and  the  air  in  the  water  in  the  soil.  Air,  in  common 
with  all  gases,  expands  very  greatly  with  a  small  in- 
crease in  temperature,  and  it  thus  exerts  a  pressure 
which  may  force  water  out  of  the  soil  into  the  larger 
drainage  channels.  Conversely,  a  lowering  of  the  temper- 
ature contracts  the  air,  and  causes  water  to  be  sucked 
into  the  soil. 

In  the  same  way,  barometric  changes  affect  the  drain- 
age of  soils.  Alternate  periods  of  low  and  high  pressure 
sweep  over  the  country  at  intervals  of  a  few  days  apart, 
and  the  changes  in  volume  of  the  outer  air  are  trans- 
mitted to  the  air  in  the  soil,  which  expands  or  contracts 
and  tends  to  draw  water  into  the  soil,  or  forces  it  out 
as  the  pressure  is  decreased  or  increased.  The  suctional 
effect  of  winds  may  have  a  similar  effect.  Strong  winds 
considerably  modify  the  air  pressure,  and  where  this 
is  brought  to  bear  on  the  soil  through  a  tile  drain  or 
other  underground  channel  it  increases  the  flow  of 
water. 

Water  does  not  necessarily  percolate  vertically  into 
the  soil.  It  may  flow  off  nearly  horizontally,  depending 
on  the  character  of  the  soil  and  its  conditions.  A  hard 


CAPILLARY   MOVEMENT  169 

subsoil  will  deflect  its  movement.  Entrapped  air  will 
do  the  same  thing,  and  this  has  been  found  to  be  a 
potent  source  of  contamination  of  open  wells  with  shal- 
low curbing.  This  is  particularly  true  in  heavy  soils, 
where  the  escape  of  entrapped  air  is  especially  difficult. 
One  of  the  beneficial  effects  of  under  drains  is  that  they 
facilitate  the  entrance  and  movement  of  rain-water 
in  the  soil  by  affording  a  channel  for  the  escape  of 
entrapped  air.  (See  page  241.) 

81.  Capillary,    or  film    movement. — Capillary  water 
lias  been  described  (see  page  141)   as  occurring  in  the 
soil  in  a  thin  film  overspreading  the  particles,  and  thick- 
ened into  a  waist-like  form  at  their  points  of  contact. 
Toward  the  bottom  of  any  soil  column  the  film  is  always 
thicker  than  at  the  top,  owing  to  the  less  weight  which 
the  surface  tension  must  bear.   This  form  of  distribution 
has  given  rise  to  the  term  film  water,  from  which  is 
derived  the  idea  of    film   movement,   to   describe  this 
type  of  capillary   movement. 

Film  movement  expresses  very  accurately  the  actual 
condition  of  affairs,  for  if  there  is  any  translocation  of 
water  at  this  stage  it  must  be  through  this  film. 

82.  Principles    governing    capillary    movement. — It 
will  be  remembered  (page  147)  that,  when  equilibrium 
is  established  in  any  mass  of  wet  soil  short  of  saturation, 
the  water  surfaces  are  comparable  to  a  stretched  elastic 
membrane.    The  more  closely  this  film  is  drawn  about 
the  particles,  the  more  surface  there  is  exposed,  and  the 
greater  pull  the  surface  tension  exerts.    Consequently 
the  greater  the  amount  of  water  which  will  be  retained. 

In  a  soil   capillarily  saturated  with   water  there  is 


170          THE   PRINCIPLES   OF  SOIL  MANAGEMENT 

no  movement.  For  the  pull  at  any  one  point  is  balanced 
by  the  pulls  from  every  other  point,  due  to  the  surface 
curvature  of  the  film  and  to  the  weight  of  the  liquid. 
In  the  bottom  of  the  column,  where  the  weight  of  the 
water  acts  in  conjunction  with  the  curvature  of  the  film, 
the  curvature  is  less  than  at  the  top  of  the  column, 
where  the 'only  effective  pull  is  due  to  the  curvature  of 
the  water  surfaces.  This  may  be  illustrated  by  the  fol- 
lowing diagram.  (Fig.  52.) 

P  represents  soil  particles  carrying  their  maximum 
film  of  water,  and  therefore  in  equilibrium  at  every 
point,  so  that  no  movement  may  take  place.  The  force 
or  pull  exerted  by  the  film  at  the  different  points  is 
represented  by  the  arrows  at  A,  B,  C,  D,  E,  etc.,  the 
length  of  the  arrow  being  proportional  to  the  pull  exerted 
by  the  film,  and  in  the  same  direction,  or  toward  the 
center  of  curvature  of  the  surface.  The  difference 
in  the  pull,  and  therefore  the  length  of  the  arrows  at 
the  top  and  bottom,  is  compensated  by  the  weight  of 
the  water  at  the  bottom.  If  water  is  now  taken  from  the 
film  into  the  rootlet  at  R,  the  curvature  of  the  film  at 
that  point  will  be  increased.  Therefore  it  will  exert  a 
greater  pull  than  the  curvatures  in  the  other  spaces, 
and  water  will  be  moved  to  R  along  the  lines  U,  to 
replace  that  taken  in  by  the  root.  So  that  the  new 
adjustment  would  be  represented  by  the  dotted  lines 
which  show  the  new  curvature  assumed  at  each  point, 
when  equilibrium  is  reestablished,  and  the  water  comes 
to  rest.  If  water  continues  to  be  lost  to  the  root,  or  by 
evaporation  from  the  soil  at  R,  the  movement  of  water 
to  that  point  will  be  continuous  as  long  as  movement  is 


CAPILLARY   ADJUSTMENT 


171 


III 


Fio.  52.  Showing  the  distribution  of  water  around  a  group  of  soil  particle*, 
and  the  distribution  of  forces  and  direction  of  movement  in  the  re-establish- 
ment of  equilibrium  after  the  removal  of  water  by  a  rootlet.  For  further 
explanation  sec  text. 

possible;  the  curvatures  meanwhile  increasing,  and  the 
films  become  thinner  and  thinner. 

It  will  lie  noted  that  the  curvature  at  every  point 
in  the  plane  1  is  the  same,  and  that  a  similar  uniformity 
prevails  for  planes  2  and  3.  Likewise,  in  the  columns 
I,  II  and  III  the  relative  curvatures  are  the  same. 


172        THE  PRINCIPLES    OF    SOIL    MANAGEMENT 

Theoretically,  therefore,  there  is  no  limit  to  which 
this  adjustment  might  take  place  in  the  horizontal  plane. 
Water  might  be  moved  in  from  a  distance  of  one  inch 
or  one  rod.  Vertically,  however,  there  would  be  a  limit 
to  the  height  to  which  water  could  be  lifted,  because 
of  the  limit  to  the  pull  of  the  surfaces  in  plane  1. 

The  larger  the  number  of  curves,  the  greater  the  total 
pull  per  unit  area,  and  consequently  the  higher  could 
water  be  lifted — just  as  there  is  a  definite  limit  to  which 
water  will  rise  in  glass  tubes  of  different  sizes.  It  is 
therefore  possible  to  keep  trimming  off  the  upper  end 
of  a  column  of  soil,  whose  lower  end  dips  in  water,  until 
the  maximum  height  through  which  water  may  be  lifted 
and  lost  by  evaporation,  or  otherwise,  is  determined. 
This  is  the  maximum  capillary  efficiency  of  the  soil,  or 
the  maximum  height  to  which  it  could  deliver  water. 

According  to  the  above  propositions,  the  movement 
of  water  would  go  on  freely  and  uniformly  until  the  mini- 
mum thinness  of  film  was  reached.  This  free  movement 
is  modified,  however,  by  another  condition.  Water, 
in  moving  from  any  point,  as  C  to  R,  must  pass  through 
the  thin  part  of  the  film  between  the  points  of  contact, 
and  where  it  comes  in  close  contact  with  the  soil  sub- 
stance. In  this,  friction  is  developed,  and  the  thinner 
the  film,  and  the  closer  it  is  drawn  about  the  particle, 
the  greater  does  this  friction  become  until  it  all  but  stops 
movement. 

For  a  period,  when  the  film  is  thick,  the  movement 
is  relatively  free;  but,  after  the  water  comes  within  the 
range  of  great  attraction  of  the  particle,  the  friction 
increases  rapidly,  and  therefore  the  movement  of  water 


IMPORTANCE   OF  CAPILLARY   MOVEMENT         173 

is  correspondingly  cut  down.  This  factor  of  friction 
greatly  limits  the  effective  capillary  capacity  of  a  soil 
both  vertically  and  horizontally.  If  the  coefficient  of 
friction  is  great,  it  will  soon  overcome  the  pull  due  to 
curvature,  and  water  will  be  quickly  moved  in  from 
only  a  short  distance.  In  proportion  as  the  friction 
coefficient  is  reduced,  the  range  of  movement  is  ex- 
tended. It  should  be  noted  that  friction  retards  move- 
ment rather  than  stops  it.  The  greater  the  surface 
over  which  a  given  volume  of  water  is  spread,  the  slower 
therefore  will  be  its  movement.  (See  page  183.) 

In  the  above  discussion  it  was  assumed  that  the 
water  is  uniform  in  all  its  properties,  and  therefore 
that  corresponding  curvatures  were  the  same.  If,  how- 
ever, anything  modifies  the  surface  tension  of  the  liquid 
at  one  point — as  change  of  temperature,  solution,  etc.,— 
this  would  be  expected  to  disturb  the  balance,  and  result 
in  film  movement.  Such  is  the  case,  as  later  examples 
will  show.  (See  page  183.)  It  is  probable  that,  in  the 
soil,  equilibrium  is  never  established,  because  of  these 
disturbing  variations  all  through  the  soil  mass.  Further, 
the  last  end  of  the  process  of  adjustment  is  exceedingly 
slow,  and  probably  never  actually  takes  place;  because 
the  force  producing  the  motion  is  successively  reduced 
as  equilibrium  is  attained,  and  because  the  difference  in 
curvature  of  the  films  is  so  slight. 

83.  Extent,  rate  and  importance  of  capillary  move- 
ment.— Capillary  movement  of  water  is  of  great  conse- 
quence to  growing  plants.  Since  it  concerns  the  capillary 
water,  it  affects  that  form  of  soil  water  upon  which 
ordinary  crops  are  directly  dependent.  The  withdrawal 


174 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


of  water  at  any  point  by  a  rootlet  is  made  up  by  move- 
ment of  water  from  the  adjacent  soil  zones.  But  the 
plant  is  not  dependent  entirely  on  the  movement  of 
water  to  its  roots.  The  roots  are  themselves  constantly 
pushing  into  fresh  soil  zones,  where  the  moisture,  and 
perhaps  also  the  food,  have  not  been  so  thoroughly 


FIG.  53.     Penetration  of  root-hairs  through  the  soil,      (h,  h')  root-hairs;   (T) 
soil  particles;    (s.  j)  air-spaces.     Water  is  indicated  by  concentric  lines. 

withdrawn.  The  roots  go  to  meet  the  capillary  advance 
of  the  soil  water.  This  advance  of  the  fine  rootlets  is 
rapid,  and  of  great  consequence  in  the  nourishment 
of  the  plant.  It  also  enables  the  roots  to  come  into 
more  intimate  contact  with  the  soil;  for,  as  the  water 
is  extracted,  it  is  lost  first  and  most  readily  from  the 
large  pores.  The  latter  amount  of  water  is  found  in 
the  smaller  spaces,  and  consequently  the  roots  are 


CAPILLARY    MOVEMENT   AND    TEXTURE 


175 


led  toward  these   small  pores   by  their   attraction   for 
water. 

Three  primary  soil  factors  govern  the  capillary 
movement  of  water.  These  are:  (a)  Texture,  (6)  struc- 
ture, (c)  dampness  of  the  soil.  In  addition  to  these, 
the  movement  is  affected  by  (d)  the  surface  tension 
of  the  soil  water,  and  (e)  by  the  condition  of  the  surfaces 
of  the  soil  particles. 


FIG.  54.    Curves  showing  the  height  and  rate  of  rise  of  water  in  dry  soils 
of  different  texture  as  given  in  Table  XXIX. 

84.  Texture. — The  influence  of  texture  was  explained 
in  the  principles  outlined  above.  The  finer  the  soil, 
the  more  surface  it  will  expose,  the  more  points  of  con- 
tact there  will  be  between  the  particles,  and  therefore 
the  greater  total  curvature  the  water  surfaces  will  have. 
For  this  reason,  a  clay  containing  20  per  cent  may  draw 
water  from  a  sand  containing  10  per  cent  of  water. 

The  capillary  capacity  of  a  soil  may  be  measured 
in  two  ways:  (1)  By  the  height  to  which  water  will 


176 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


be  raised  in  soils  of  different  texture.  (2)  By  the  total 
amount  of  water  raised  through  a  given  height  in  a 
definite  time.  The  time  element  enters  into  both  sorts 
of  measurements,  and  is  an  especially  important  con- 
sideration in  clay  soil  where  the  movement  is  generally 
very  slow. 

TABLE  XXIX 

SHOWING  HEIGHT  OF  RISE  OF  WATER  IN  DRY  SOILS  OF  DIFFERENT 
TEXTURE,  AS  SHOWN  IN  THE  ABOVE  CURVES 


Time 

Kin. 

Hours 

Days 

15 

1 

2 

1 

3 

8 

13 

19 

Inches 

Inches 

Inches 

Inches 

Inches 

Inches 

Inches 

Inches 

1.  Silt  and   very 

fine  sand  .  .  . 

2.7 

4.7 

7.0 

20.0 

30 

45.0 

52.0 

56.0 

2.  Very  fine  sand 

7.6 

10.0 

12.4 

21.0 

23 

2ti.O 

27.5 

28.5 

3.  Fine  sand  

9.0 

9.5 

10.0 

11.6 

13 

14.3 

15.2 

16.0 

4.  Coarse  and  me- 

dium sand  .  . 

5.8 

6.0 

6.3 

7.5 

9 

10.0 

11.5 

12.5 

5.  Fine  gravel  .  .  . 

4.0 

5.0 

5.3 

6.4 

8 

9.0 

10.0 

10.8 

These  materials  were  sifted  to  fairly  uniform  sizes, 
according  to  the  scale  given  above  (page  73).  The  silt 
was  a  natural  material,  containing  a  large  amount  of 
very  fine  sand,  together  with  some  clay.  It  might  be 
termed  a  light  silt  loam.  It  will  be  particularly  noted 
that  the  smaller  classes  of  particles — silt  and  cla^— 
have  a  relatively  large  influence  on  capillary  movement. 
Above  the  class  of  fine  sand,  there  is  not  much  variation 
in  the  height  of  rise  for  different  textures,  the  total 
height  attained  being  slight. 


CAPILLARY   MOVEMENT   AND    TEXTURE 


177 


Loughridge  has  made  very  careful  determinations 
of  the  capillary  power  of  four  dry  soils  of  known  physical 
composition  over  a  period  ranging  from  6  to  195  days. 
These  soils  range  in  texture  from  light  sandy  loam  to 
heavy  clay  adobe  of  the  following  mechanical  com- 
position. 

TABLE  XXX 


Per  cent  of  each  separate  present 

Clay 
leas  than 

Fine  silt 

Coarse 
silt 

Sand 

Limits  used  in  diameter  of  particles 

.01 
mm. 

.01  -.025 
mm. 

.025-  .047 
mm. 

.04  7-  .5 
mm. 

1    Sand  soil    

2.82 
3.21 
15.02 
44.27 

3.03 
5.53 
15.24 
25.35 

3.49 
15.42 
25.84 
13.47 

89.25 
72.05 
45.41 
13.37 

2    Light  sandy  loam  

3    Silty  loam   

4    Clay  soil  

The  composition  of  these  soils  is  also  shown  in  the 
following  curves: 


*UI 

V 

a  72 

°o 

"mi 
mi 

V         o_ 
x       °o 
""i      °        1 

S  48 

X     °0       * 
X    °0 
y£^^— 

• 
u    3C 

•----.    »•«. 
-    *L 

_^-  —  "^ 

t  84 

V- 

-V.ffl.Tlr  t04M 

^^&* 

£  •  < 

0      X 

Tjizi—  «rcr. 

.  

s" 

I 

°°J 

rrgS    £**£££ 

SANDS 
HH7.  -.6 


COARSE  SILT 
(.026  -.M7  UK.) 


FINE  SILT 
(.01  -  .026  M« 


FIG.  55.  Curves  showing  the  mechanical  composition  of  the  soils  whose 
analysis  is  given  in  Table  XXX,  and  whose  capillary  water  capacity  is  given  in 
Table  XXXI  and  Figs.  56  and  57. 


178 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


The  capillary  rise  of  water  in  these  soils  was  as  fol- 
lows: 

TABLE  XXXI.— TIME  IN  HOURS 


Min. 

Hours 

30 

1 

2 

6 

12 

Height  of  rise  in  inches 

1.  Sand  

8.0 
9.0 

2.7 
1.4 

10.0 
12.7 
4.8 
2.5 

12.0 
19.0 
8.8 
5.0 

13 
24 
11 

8 

2.  Light  sandy  loam  

6.3 

3.  Silty  loam  

4.  Clay   

0.8 

TABLE  XXXI,  continued. — TIME  IN  DAYS 


Days 


l 

2 

6 

12 

26 

48 

90 

160 

195 

Height  of  rise  in  inches 

1.  Sand    

14.0 

28.2 
13.0 
10.0 

15.5 

30.5 
17.0 
14.0 

17.0 

35.0 
20.5 
20.0 

38 
25 
23 

41 
31.5 
26.5 

44 
35 

46.5 
40.0 

45 

50 
46 

2.  Light    sandy 
loam  

3.  Silty  loam  

4.  Clay  

As  is  always  the  case,  the  rise  is  most  rapid  immedi- 
ately after  the  soil  is  placed  in  contact  with  the  water 
and  the  rate  of  rise  decreases  progressively  as  the  limit 
is  reached.  The  more  coarse  the  texture  of  the  material, 
the  more  quickly  is  the  limit  of  rise  attained. 

These  figures  are  shown  in  the  following  curves:  (1) 
For  the  first  twelve  hours;  (2)  for  the  full  period. 


CAPILLARY   MOVEMENT  AND   TEXTURE 


179 


.000 


t-.SAND 


2 


8 


10        11 


12 


4667 
TIME  IN  HOURS 

Fio.  56.   Curves  showing  the  capillary  rise  of  water  in  twelve  hours  in  dry 
soils  of  different  texture  as  given  in  Table  XXXI. 


60 

46 

S40 
o36 

Z30 


o  16 
S10 


ao 


40 


eo 


140 


160        180        200 


00         100         120 

TIME  IN  DAYS 

Fio.  57.   Curves  showing  the  capillary  rise  of  water  in   195  days  in  dry  soils  of 
different  texture  as  given  in  Table  XXXI. 

It  is  evident,  from  a  very  simple  laboratory  experi- 
ment, that  the  rate  of  capillary  movement  in  dry  soil 
is  inversely  proportional  to  the  fineness  of  texture, 
while  the  total  height  of  rise  is  directly  related  to  the 


180          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

texture.  Up  to  a  height  of  three  feet,  the  sandy  loam 
moves  water  most  quickly.  But  when  it  is  necessary 
to  move  water  to  a  greater  height,  a  finer  soil  is  required. 
The  maximum  height  traversed  by  capillary  water 
in  the  above  soils  is  fifty-six  inches  in  the  silt  loam, 
and  in  two  instances  this  appeared  to  be  near  the 
limit  of  capillary  efficiency  in  dry  soil  of  that  texture. 
In  clay  the  movement  goes  on  very  slowly,  and  an 
excessively  long  period  is  required  for  the  limit  to  be 
reached.  A  crop  might  perish  of  drought  before  water 
would  move  up  to  meet  its  needs.  That  is,  in  fine-tex- 
tured soil,  although  its  capillary  capacity  is  very  great, 
the  surface  area  of  the  particles  is  so  large,  and  therefore 
the  friction  in  the  movement  of  water  so  great,  that  the 
actual  capillary  movement  is  very  inefficient. 

The  same  fact  appears  here  that  appeared  in  the 
discussion  of  the  water  capacity  of  soils,  namely,  that 
it  is  the  soil  of  intermediate  texture — the  silt  and  fine 
sandy  loam- — which  most  readily  meets  the  needs  of 
crops  for  water. 

85.  Dampness  of  soil  particles. — The  capillary  move- 
ment in  dry  soil,  as  given  above,  does  not  represent 
the  true  capillary  capacity.  When  a  soil  is  dry,  it  has 
been  shown  that  it  resists  wetting,  and  therefore  resists 
the  capillary  rise  of  water.  Natural  field  soils  always 
contain  some  oily  substances,  which  are  deposited  on 
the  surface  of  the  soil  particles  when  the  soil  is  dried. 
This  oily  matter  retards  greatly  the  wetting  of  the  par- 
ticles, which  takes  place  only  after  this  material  has 
been  again  dissolved,  so  that  a  clean  surface  is  exposed 
to  the  solution.  Therefore,  in  a  soil  which  already  con- 


CAPILLARITY   IN   MOIST   AND   DRY   SOILS 


181 


tains  a  small  amount  of  water,  it  is  to  be  expected  that 
the  total  capillary  rise,  and  the  rate  of  movement  of 
water,  would  be  most  rapid.  That  is,  a  slight  dampness 
of  the  soil  is  conducive  to  the  most  rapid  capillary 
movement.  Briggs  found  the  limit  of  capillary  move- 
ment in  dry  Sea  Island  soil — light,  fine,  sandy  loam — 
to  be  about  36  centimeters,  or  15  inches,  in  14  days; 
while,  in  the  same  soil  in  a  moist  condition,  water  was 
raised  through  a  column  165  centimeters,  or  66  inches, 
in  height — 4.5  as  great  a  height  as  in  the  dry  soil.  Stew- 
art found  the  following  limits  for  three  sands  of  slightly 
different  texture  when  dry  and  wet. 

TABLE  XXXII 


Dry 

Wet 

Soil  No.  1  .  . 

Inches 

31.8 

Inches 
112.5 

Soil  No.  2  

58.1 

141.8 

Soil  No.  3  

86.8 

174.1 

These  results  are  in  accord  with  field  experience. 
The  figures  for  the  moist  soil  most  nearly  represent 
the  heights  to  which  soils  raise  water,  and  further,  under 
field  conditions,  the  soil,  with  the  exception  of  the  immedi- 
iate  surface,  seldom  becomes  air  dry  in  the  humid 
regions.  Consequently,  capillary  movement  concerns 
chiefly  moist  soils. 

There  are  two  factors  operative  to  prevent  capillary 
distribution  from  moist  to  dry  soil.  One  of  them  is 
resistence  to  wetting.  The  other  is  the  very  slow  move- 
ment of  water  in  thin  capillary  films, — that  is,  when 


182         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

it  is  reduced  to  near  the  minimum  capillary  content, 
or  wilting  point.  At  this  stage  the  movement  is  exceed- 
ingly slow,  because  of  the  excessive  friction.  This  funda- 
mental principle  is  made  use  of  in  soil  mulches  and  de- 
termines their  usefulness,  and  should  direct  their  man- 
agement. (See  page  203.) 

86.  Structure. — Structure  affects  capillary  movement. 
It  has  been  shown  how  capillary  movement  is  largely 
due  to  the  size  of  the  individual  spaces  in  the  soil. 
The  size  of  the  spaces  is  due,  (1)  To  the  size  of  the  par- 
ticles.     (2)  To    their    arrangement.      (See    page    203.) 
The  smaller  the  particles,  and  therefore  the  smaller  the 
pores,  the  greater  the  capillary  power  and  the  slower 
the  movement.    In  so  far  as  the  arrangement  of  the 
particles  or  structure  effects  a  change  in  the  effective 
size  of  the  pores,   it   affects  the  capillary   movement. 
In  a  puddled  structure  the  movement  is  much  more 
slow  than  in  a  soil  having  a  granular  or  crumb  structure. 
Any   tillage   operation   which    alters   the   structure,    in 
either  one  direction  or  the  other,  thereby  alters  the 
capillary  power  and  the  rate  of  movement.    Compact- 
ing a  soil  is  well  known  as  a  process  which  seems  to 
draw  moisture  into  the  compacted   zone;  while  culti- 
vation, or  loosening  the  soil  structure,  has  the  opposite 
effect.     Upon    this    fact    are    based    many    important 
tillage  operations,  such   as  rolling  after   seeding  small 
grains. 

87.  Surface  tension. — Surface  tension  affects  capillary 
movement  in  the  same  way  that  it  affects  the  capillary 
retention  of  water.    It  represents  the  cohesive  properties 
of  the  liquid,  and  corresponds  to  an  elastic  membrane. 


CAPILLARY   MOVEMENT   AND  STRUCTURE         183 

The  stronger  such  a  membrane,  the  larger  the  pull  it  can 
exert  under  a  given  strain.  Consequently,  in  a  soil  of 
uniform  texture,  and  in  moisture  equilibrium,  any- 
thing which  changes  the  surface  tension  may  set  up 
motion  of  the  soil  water.  The  introduction  of  fertilizers 
may  set  up  such  a  movement,  and  this  addition  to  a  soil 
may  enable  it  to  draw  and  permanently  retain  more 
water  than  the  adjacent  soil  of  same  texture.  Appli- 
cations of  magnesium  chloride,  salt  and  muriate  of 
potash,  are  observed  to  keep  the  soil  more  moist  in  dry 
weather,  and  a  similar  effect  of  some  alkali  salts  has 
been  noted.  These  materials  all  raise  the  surface  tension. 
High  temperature  reduces  the  surface  tension,  and  there- 
fore, in  a  soil  in  moisture  equilibrium,  if  one  part,  as 
the  surface,  is  heated,  the  water  will  be  drawn  away 
from  that  region  to  the  cooler  zone,  where  the  tension 
is  higher. 

88.  Condition  of  surfaces  of  particles. — The  condition 
of  the  surface  of  the  soil  particles  affects  the  tenacity 
with  which  water  adheres  to  them.  The  application 
of  oil  to  a  soil  tends  to  destroy  its  capillary  capacity; 
and  any  substance  in  the  soil  which  will  bring  about 
such  a  condition  reduces  the  capillary  efficiency  of  the 
soil.  ' 

The  action  of  capillarity  is  not  limited  to  any  one 
direction.  It  may  take  place  in  any  direction.  It  has 
usually  been  measured  vertically  upward.  But  it  oper- 
ates vertically  downward,  as  well,  and  it  moves  water 
horizontally.  The  vertical  upward  movement  of  capillary 
water  is  modified  by  the  influence  of  gravity,  as  is  capil- 
lary retention.  (See  page  149.) 


184 


THE   PRINCIPLES   OF  SOIL  MANAGEMENT 


The  following  curves  show  the  capillary  transfer  of 
water  in  two  soils  through  eight  feet  horizontally. 


END  EXPOSED  TO  EVAPORATION 
PERCENT.  OF  MOISTURE 
o  M  »  e>  oo  o  to  .*  os  o> 

iii 

c,\A 

LiS^M 

pTfc*v^_ 

JNITJAL-PI 

RCENTr-OF 

WATER  — 

> 

^^" 

—  

/ 

SAND 

AFTER 

70  DAYS 

/ 

/ 

/ 

/  z 

'/ 

1 


28466 
DISTANCE  FROM  EXPOSED  END,   IN  FEET 

FIG.  58.  Curves  showing  the  initial  moisture  content  of  horizontal  columns 
of  sand  and  clay  loam  soil  and  the  distribution  of  moisture  after  free  evapora- 
tion from  one  end  of  each  column,  for  a  period  of  days.  Note  the  general 
movement  of  water  throughout  the  columns. 

It  is  evident  that  plants  may  make  use  of  supplies 
of  moisture  to  one  side,  as  well  as  below  their  roots  even, 
in  some  soils,  to  a  distance  of  several  feet,  through  the 
agency  of  capillarity.  On  the  other  hand,  irrigation 
farmers  have  repeatedly  noted  the  very  limited  lateral 
influence  upon  crops  of  the  application  of  water  in  irri- 
gation. The  limit  of  the  application  of  water  is  in  some 
soils  marked  almost  to  the  row.  In  this  instance,  it 
should  be  remembered  that  water  is  added  only  after 
the  soil  has  become  relatively  dry,  at  which  stage  the 
moisture  films  move  with  great  difficulty  due  to  friction, 
and  probably  also  to  cracks,  which  of  course  very  effect- 
ively break  up  capillarity.  King  has  concluded  from 
studies  on  vertical  columns  that  an  adjustment  of  water 
through  ten  feet  of  soil  may  readily  take  place. 


AMOUNT   OF   WATER    MOVED  185 

89.  Examples  of  the  amount  of  water  moved. — In 
crop  production,  the  crucial  test  of  the  capillary  capacity 
of  the  soil  is  the  amount  of  water  it  is  able  to  move. 
It  must  not  only  be  able  to  move  water  a  long  distance, 
or  to  a  great  height,  but  it  must  be  able  to  move  a  rela- 
tively large  amount  of  water,  and  to  move  it  quickly 
if  the  movement  shall  be  effective.  The  important 
consideration  is  the  amount  of  water  moved  a  given 
distance  in  a  given  time.  A  soil  may  be  able  to  quickly 
move  large  volumes  of  water  to  a  height  of  a  foot, 
and  l)e  utterly  ineffective  to  a  height  of  five  feet.  On 
the  other  hand,  a  soil  may  be  so  fine  as  to  be  able  to 
lift  water  to  a  height  of  forty  feet,  and  yet  the  move- 
ment be  so  slow  and  the  amount  of  water  moved  be 
so  small  that  the  result  is  negligible, — that  is  the  soil 
is  capillarily  ineffective.  It  therefore  appears  that, 
for  any  given  distance  within  reason  and  for  any  normal 
moisture  demand  of  a  crop,  there  is  a  texture  and  struc- 
ture of  soil  which  will  most  readily  meet  those  demands. 
If  the  water-table  is  three  feet  below  the  surface,  a 
very  coarse  soil  may  suffice.  If  the  water-table  is  ten 
feet  below  the  surface,  a  much  finer  soil  will  be  necessary. 
On  the  other  hand,  to  supply  a  full-sized  pumpkin  vine, 
having  a  large  evaporation,  from  a  water  supply  five 
feet  away,  will  require  a  finer  soil  than  is  required 
to  supply  a  Jersey  pine  having  a  small  evaporation. 
In  other  words,  we  need  to  know  the  effective  capillary 
capacity  of  each  soil  to  different  heights  and  distances, 
up  to  their  limits. 

Very  little  data  of  this  sort  is  available.  None  is 
available  for  horizontal  movement,  and  the  figures 


186 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


on  vertical  movement  are  very  incomplete  and  inade- 
quate. King  made  such  a  study  of  sifted  quartz  sand 
having  a  mean  diameter  of  .47  mm.,  by  means  of  a 
column  with  an  expanded  top,  and  found  that  the  sand 
was  able  to  raise  water  to  a  height  of  6.75  inches  at 
the  rate  of  44  inches  of  water  per  day,  equivalent  to 
1,340  feet  per  year.  But  this  same  sand  failed  to  lift 
any  appreciable  amount  of  water  to  a  height  of  11.75 
inches.  King  has  also  found  the  following  movement 
to  take  place  to  different  heights  in  columns  of  soil 
one  square  foot  in  cross  section,  where  the  loss  was 
measured  by  evaporation  from  the  surface. 

TABLE  XXXIII 


Height  in  feet 

1  foot 

2  feet 

3  feet 

4  feet 

f£ 

i*3i 

« 

&0 

u 

So) 

ft* 

|g 

r  03 

2U 

h 

r  as 

B 

8  S 

•a! 

•g* 

*j 

•s^ 

^2 

•i* 

^z 

•s  ^ 

1 

P 

££ 

51 

§£ 
l| 

Jl 

§& 
cSS? 

a  - 

5  8, 

§& 

o>, 
ft  3 

•si 

•a 

•3 

T3 

T3 

1.  Fine  quartz 

sana  

2.37 

166 

207 

146 

1  23 

86 

91 

64 

2.  Clay  loam  .... 

2.05 

144 

1.62 

113 

1.00 

70 

.90 

63 

As  remarked  by  Professor  King,  these  figures  prob- 
ably do  not  represent  the  maximum  capacity  of  these 
soils  to  the  heights  stated.  The  shorter  the  column, 
the  less  accurate  are  the  figures.  For  in  the  short  col- 
umns the  evaporation  was  correspondingly  less  than 
the  movement.  From  the  results,  it  appears  that  the 


AMOUNT   OF   WATER    MOVED 


187 


clay  soil  was  in  a  very  well-granulated  condition,  which 
brings  its  rate  very  near  that  of  the  sand.  It  also  appears 
from  this  data,  as  was  shown  in  the  data  on  height 
and  time  of  capillary  rise,  that,  up  to  three  or  four  feet, 
the  fine  sand  is  as  efficient  as  the  soil  of  much  finer 
texture. 

In  studies  on  the  capillary  rise  of  water  in  moist  Sea 
Island  cotton  soil — a  fine  sandy  loam, — Briggs  found 


Fir,.  59.     The  weeder,  with  riding  attachment.    For  very  shallow  cultivation 
in  mellow  soil  free  from  stone  and  rubbish. 

the  movement  to  be  at  the  rate  of  1.3  pounds  per  square 
foot  per  day,  or  91  inches  per  year,  through  a  height 
of  85  centimeters  (34  inches).  But  when  the  column 
of  the  sand  soil  was  105  centimeters  long,  water  was 
raised  at  the  rate  of  .32  of  a  pound  per  square  foot  per 
day,  or  21.4  inches  per  year, — a  decreased  efficiency 
from  doubling  the  height  of  the  column  of  75.4  per  cent, 
When  the  column  was  185  'centimeters  in  height,  no 
appreciable  loss  took  place, — indicating  that  this  sand 


188 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


was  not  able  to  raise  water  to  the  height,  even  when 
moist. 

Buckingham  obtained  the  following  results,  which 
show  a  very  considerable  vertical  movement  in  fine 
sandy  loam  soils  to  a  height  of  nearly  four  feet. 

TABLE  XXXIV 


I 

II 

III 

IV 

Height  of 

Dry 

Pounds  of 

Inches  of 

column  . 
Inches 

porosity 

water  per  day 
per  sq.  ft. 

water  per 
year 

1.  Takoma 

lawn  

46 

48 

.73 

51.6 

2.  Podunk 

fine  sandy 

loam  .... 

46 

35 

.56 

39.4 

It  must  be  kept  in  mind,  in  examining  these  figures, 
that  the  evaporation  conditions  in  the  different  experi- 
ments were  not  uniform,  and  therefore,  that  the  results 
are  not  strictly  comparable.  They  do,  however,  show 
the  movement  of  a  very  large  amount  of  water  in  this 
way  through  distances  of  several  feet.  The  amount 
so  moved  in  these  sandy  soils  per  year  is  several  times 
the  total  amount  required  to  produce  normal  crops. 
(See  page  134.)  There  is  also  indication  that  in  the  short 
period  of  a  day  the  amount  of  water  moved  is  sufficient 
to  meet  the  needs  of  a  considerable  mass  of  growing 
plants.  It  is  regrettable  that  no  figures  are  available 
for  silt  and  clay  soils,  and  to  greater  heights  and  hori- 
zontal distances,  in  order  that  a  more  complete  idea 
of  the  availibility  of  water  supplies  at  a  distance  of  six, 


THERMAL    MOVEMENT    OF    WATER  189 

eight  and  ten  feet,  or  even  more,  may  be  had.  This 
is  an  important  body  of  information  yet  to  be  gained. 

90.  Thermal  movement. — Water  moves  through  the 
soil  in  the  form  of  vapor.  If  a  glass  vessel  or  tube  filled 
with  moist  soil  be  set  on  a  hot  surface,  the  bottom  of  the 
column  will  be  seen  to  become  lighter  in  color,  indicating 
a  loss  of  moisture.  If  the  whole  column  is  not  heated 
and  the  moisture  is  determined  in  successive  sections, 
beginning  at  the  top,  or  coldest  portion,  the  moisture 
content  will  be  found  greatest  a  short  distance  above 
the  heated  layer  at  the  bottom. 

When  the  moist  soil  is  heated,  steam  is  formed,  which 
develops  a  pressure  that  forces  the  vapor  rapidly  through 
the  soil.  But,  at  ordinary  temperatures,  this  vapor 
movement  is  the  result  of  simple  diffusion,  and  it  obeys 
the  same  laws.  Buckingham  has  shown  that  the  diffu- 
sion of  air  through  the  pores  of  the  soil  is  exceedingly 
slow,  and  therefore  that  this  phase 'of  soil  aeration  is 
of  small  effect.  (See  page  439.)  He  has  also  shown 
that  the  diffusion  of  water  vapor  through  the  fine  pores 
of  the  soil  is  very  slow.  (See  table  below.) 

It  is  well  known  that  water  does  not  necessarily 
evaporate  at  the  surface  of  the  soil.  It  may  evaporate 
in  the  deep  pores  in  the  soil  if  the  air  at  that  point 
is  sufficiently  dry.  Atmosphere  in  a  moist  soil  is  very 
near  saturation.  In  a  mulched  soil  (see  page  199)  evapo- 
ration may  take  place  at  the  top  of  the  moist  layer. 
The  loss  of  water  will  therefore  depend  very  largely 
upon  the  loss  of  moisture  by  diffusion  through  the 
mulch.  Buckingham  obtained  the  interesting  results 
given  in  Table  XXXV  bearing  on  this  point: 


190 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


TABLE  XXXV. — Loss  OF  WATER  BY  EVAPORATION  FROM  BELOW 
COLUMNS  OF  DIFFERENT  AIR-DRIED  SOILS 


Soil 

Depth  of 
soil  layer 

Initial 
porosity 

Rate  of  loss 
of  water 
per  year 

Coarse  sand    

Inches 

2 

Per  cent 
45 

Inches 
4.30 

Fine  sandy  loam  

1 

48 

2.52 

Fine  sandy  loam  

2 

46 

1.59 

Fine  sandy  loam  

4 

41 

0.93 

Fine  sandy  loam  

6 

46 

067 

Silt  loam  

1 

54 

2.71 

Silt  loam  

2 

51 

1.60 

Silt  loam  

4 

49 

0.95 

Silt  loam  

6 

51 

0.69 

Clay.. 

2 

46 

0.60 

It  appears  from  these  figures  that  the  thermal  move- 
ment of  water  by  simple  diffusion  is  determined:  (1) 
By  the  size  of  the  individual  pores.  (2)  By  the  total 
amount  of  pore  space  in  the  soil.  (3)  Upon  the  thickness 
of  the  soil  layer.  When  equally  dry  the  fine-textured 
soil  retains  moisture  as  vapor  more  effectively  than  does 
coarse-textured  soil.  In  so  far  as  the  structure  of  the 
soil  modifies  either  the  size  of  the  pores  or  their  total 
volume,  it  may  modify  the  loss  of  water.  A  coarsely 
cloddy  mulch  would  therefore  be  ineffective.  Particu- 
larly striking  is  the  small  depth  of  soil  which  is  effective 
to  prevent  the  loss  of  water.  Even  the  one-inch  mulch 
has  a  wonderfully  high  efficiency. 

IV.     CONTROL    OF    SOIL    WATER 


In  the  control  of  soil  moisture  it  is  desired  to  accom- 
plish one  of  two  things:  (a)  The  average  water  content 


CONTROL   OF   SOIL    MOISTURE 


191 


of  the  soil  is  increased  or,  (6)  the  average  water  content 
of  the  soil  is  decreased.  If  the  crop  is  likely  to  suffer 
from  a  deficiency  of  water,  or  from  conditions  associated 
with  a  deficiency  of  water — as  food, — we  aim  to  in- 
crease the  moisture  supply  by  conserving  the  rainfall, 
or  by  direct  additions  of  water.  On  the  other  hand, 
in  soils  saturated  with  water,  or  which  are  too  cold, 
or  too  poorly  aerated  because  of  an  excess  of  water, 
it  is  desired  to  remove  this  excess  either  by  drainage 
or  appropriate  tillage 
methods. 

91.  Means    of    in- 
creasing    the    water 
content  of  the  soil. — 
The     average     water 
content    of    the    soil 
may  be  increased  in 
three   ways:    (1)    By 
decreasing  the  losses 

from  (a)  percolation  and  (6)  evaporation.  (2)  By 
increasing  the  capacity  of  the  soil  for  water  (a)  by 
modifications  of  texture  and  structure,  and  (b)  by  in- 
creasing the  humus  content.  (3)  By  the  direct  addition 
of  water  to  the  soil,  which  is  irrigation. 

92.  Decreasing  loss. — The  water  which  comes  on  the 
soil  is  subject  to  two  forms  of  loss,    (a)   It  may  percolate 
through  the  soil  and  beyond  the  reach  of  plant  roots. 
(6)  It  may  evaporate. 

93.  Percolation. — The    amount  of    loss   in    this  way 
is  very  great.    (See  page  192.)    Water  percolates  most 
rapidly  in  large  spaces,  and  whether  these  large  spaces 


FIG.  60.  One  -  row  toothed  cultivator. 
Adapted  to  shallow  tillage  and  the  mainte- 
nance of  a  mulch. 


192 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


are  the  result  of  coarse  texture  or  of  a  loose,  cloddy 
structure,  the  final  result  is  the  loss  of  water.  The  fol- 
lowing table  shows  the  average  results  of  the  Rothamsted 
drain  gages  for  thirty-four  years,  by  months  from  1871 
to  1904,  on  a  rather  heavy  loam  or  clay  loam  soil,  and 
twenty,  forty  and  sixty  inches  in  depth.  These  gages 
have  an  area  of  one  thousandth  of  an  acre  each,  and  are 
kept  free  from  vegetation. 

TABLE  XXXVI 


Rain- 
fall 

Drainage  through 
soil 

Proportion  of  rainfall 
drained  through  soil 

20 

40 

60 

20 

40 

60 

Depth  in  inches 

Per  cent 

January      .    . 

2.32 
1.97 
1.83 
1.89 
2.11 
2.36 
2.73 
2.67 
2.52 
3.20 
2.86 
2.52 

1.82 
1.42 
0.87 
0.50 
0.49 
0.63 
0.69 
0.62 
0.88 
1.85 
2.11 
2*.02 

2.05 
1.57 
1.02 
0.57 
0.55 
0.65 
0.70 
0.62 
0.83 
1.84 
2.18 
2.15 

1.96 
1.48 
0.95 
0.53 
0.50 
0.62 
0.65 
0.58 
076 
1.68 
2.04 
2.04 

78.5 
72.2 
47.6 
26.5 
23.2 
24.0 
25.3 
23.2 
35.0 
57.8 
76.7 
80.3 

88.4 
80.0 
55.6 
30.0 
26.1 
27.6 
25.6 
23.2 
32.8 
57.5 
76.3 
85.4 

84.5 
75.2 
52.0 
28.0 
23.6 
26.3 
23.8 
21.7 
30.0 
52.5 
72.4 
M.O 

February 

March.                .  . 

April  

May  

June    

July  . 

August    

September   

October  

November  

December  

Mean    total   per 
year.  .  , 

28.98 

13.90 

14.73 

13.79 

48.2 

51.0 

48.0 

Maximum  

Results  for  maximum  and  minimum  rainfall 

38.70 
20.50 

23.50 
7.32 

23.60 
7.90 

24.30 
7.70 

60.7      61.0 
35.7      38.5 

63.0 
37.6 

Minimum  

LOSS    BY    PERCOLATION 


193 


The  rainfall  and  relative  loss  through  gages  of  differ- 
ent depths  is  shown  in  the  following  curves,  based  upon 
the  above  figures. 


Fio.61.  Curves  representing  the  annual  rainfall  and  percolation  through 
20,  40  and  60  inches  of  soil  by  months.  Rothamsted,  England.  Average  of  34 
years. 

It  appears  from  these  figures  and  curves  that  about 
50  per  cent  of  the  rainfall  is  lost  by  percolation,  under 
the  climate  of  England.  It  also  appears  that  the  loss 
is  slightly  less  from  the  sixty-inch  than  from  the  twenty- 
inch  gage.  Under  a  climate  less  humid,  this  difference 
is  greater.  This  is  illustrated  in  two  ways:  (1)  In  the 
above  table  it  is  clear  that  the  proportion  of  water  lost 
by  drainiage  is  much  less  in  summer  than  in  winter. 
(See  page  195.)  The  saving  is  somewhat  larger  in  the 
deep  than  in  the  shallow  gage,  as  the  proportionate 

M 


194         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

capacity  of  the  soil  for  water  is  somewhat  greater  in 
this  case.  (2)  It  has  been  estimated  that  the  annual 
run-off  of  the  streams  in  the  eastern  half  of  the  United 
States  amounts  to  about  50  per  cent  of  the  rainfall; 
but  in  the  basin  of  the  Missouri  river  the  run-off  is  not 
over  20  per  cent  of  the  rainfall,  and  in  the  Great  Basin 
it  is  practically  nil. 

These  figures  give  some  idea  of  the  total  amount 
of  water  lost  by  percolation  through  the  soil,  and  repre- 
sent a  supply  which  it  is  the  aim  of  good  soil  manage- 
ment to  lessen  or  eliminate,  according  to  the  needs  of 
the  crop.  Loss  from  percolation  may  be  reduced  in  two 
ways,  which  depend  upon  the  fact  that  the  rapidity 
of  such  loss  is  directly  proportional  to  the  size  and  volume 
of  the  pore  spaces  in  the  soil.  These  are  (a)  by  modifi- 
cations of  texture,  (6)  by  modifications  of  the  structure 
of  the  soil.  The  primary  method  is,  or  course,  that 
modification  of  structure  which  breaks  down  the  granular 
arrangement  and  permits  a  greater  compactness.  When 
rain  falls  on  the  soil,  its  fall  is  not  stopped.  It  continues 
to  fall  through  the  soil  at  a  reduced  rate  as  gravitational 
water.  And,  as  the  movement  of  this  gravitational  water 
is  directly  determined  by  the  fineness  of  the  soil  spaces, 
it  is  possible  to  very  greatly  reduce  this  type  of  move- 
ment by  compacting  the  soil  structures.  The  greater 
compactness  of  the  soil  lengthens  out  the  period  during 
which  the  soil  contains  hydrostatic  water,  and,  if  the 
roots  of  growing  plants  are  distributed  through  the  soil, 
they  are  able  to  make  a  larger  use  of  this  free  water 
than  would  be  possible  if  the  wave  of  saturation,  as  a 
result  of  rainfall  or  irrigation,  quickly  passed  beyond 


LOSS    BY   EVAPORATION  195 

their  reach.  Therefore,  on  soils  subject  to  excessive 
leaching,  water  may  be  conserved  by  use  of  the  roller 
or  other  compacting  implement,  and  by  such  manage- 
ment as  permits  the  deep  subsoil  to  become  more  dense. 
94.  Evaporation. — The  second  form  of  soil-moisture 
loss  is  by  surface  evaporation.  It  has  been  shown  that, 
in  the  process  of  growth,  a  large  volume  of  water  is 
evaporated  directly  from  the  tissues  of  the  plants.  In 
this  process  it  performs  useful  functions.  But  a  large 
amount  of  water  is  also  lost  by  direct  evaporation  from 
the  surface  of  the  soil.  If  the  plants  which  evaporate 
the  soil  water  are  those  of  the  desired  crop,  the  loss  is 
proper  and  not  to  be  avoided.  But  it  frequently  happens 
that,  either  before  the  regular  crop  is  on  the  land  or 
mixed  with  it,  are  large  numbers  of  worthless  plants 
through  which  this  same  moisture  loss  occurs.  This  is 
of  course  a  waste  of  moisture,  and  is  to  be  avoided  by 
preventing  their  growth.  It  may  happen  in  the  spring 
that  the  late  plowing  of  land  bearing  a  heavy  growth 
of  vegetation  permits  so  great  a  loss  in  this  way  that, 
unless  the  subsequent  season  is  one  of  abundant  rain- 
fall, the  regular  crop  may  suffer  from  the  lack  of  moisture 
which  was  stored  in  the  soil,  and  by  timely  plowing  and 
preparation  could  have  readily  been  utilized.  In  this 
connection,  it  should  be  kept  in  mind  that  green  manure 
crops  may  be  directly  injurious  the  first  season  if  they 
are  permitted  to  grow  so  late  before  being  turned  under 
as  to  unduly  deplete  the  soil  moisture.  In  the  manage- 
ment of  green  manure  crops,  that  optimum  point  when 
the  excess  of  water  due  to  heavy  spring  rain  and  winter 
snow  has  been  removed,  but  the  capillary  supply  not 


196          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

impaired,  should  be  selected.  In  semi-arid  regions, 
where  dry  farming — farming  without  irrigation  where  it 
is  usually  required— is  practiced,  it  is  sometimes  advis- 
able to  grow  but  one  crop  in  two  years,  because  the 
annual  rainfall  is  not  sufficient  to  produce  a  profitable 
crop  each  season.  This  practice,  of  course,  implies 
those  conservation  practices  which  safeguard  the  rainfall 
as  it  collects,  by  appropriate  tillage  methods. 

The  loss  of  water  by  direct  evaporation  from  the  soil 
may  be  excessive,  and  result  in  direct  reduction  of  the 
crop  yield.  This  type  of  loss  is  so  familiar  that  examples 
hardly  need  be  cited.  In  the  results  with  the  Rotham- 
sted  rain  gages,  about  50  per  cent  of  the  annual  rainfall 
was  regained  in  the  drainage  water.  Since  the  gages 
bore  no  crop,  the  remaining  50  per  cent  must  have 
been  lost  by  evaporation.  And  it  will  be  noted  that  in  the 
summer  months  under  warm  temperature  this  loss  was 
greatest,  amounting  to  75  per  cent  of  the  rainfall. 
Correspondingly,  in  the  semi-arid  and  arid  sections  of 
the  country,  where  there  is  little  or  no  drainage,  the  rain- 
fall is  all  lost  by  evaporation.  Investigations  indicate 
that  about  70  per  cent  of  the  precipitation  on  the  land 
surface  is  derived  from  evaporation  from  the  land  sur- 
face. Even  in  the  humid  sections,  where  the  annual 
rainfall  is  ample  for  maximum  crop  production,  the  crops 
are  frequently  reduced  even  below  the  profit  point  by 
prolonged  periods  of  dry  weather  in  the  growing  season, 
during  which  the  loss  from  the  plants,  coupled  with  the 
loss  from  the  soil,  exhausts  the  soil  supply.  If  we  refer 
to  page  135,  we  note  that  the  water  absolutely  needed 
for  crop  production,  and  including  the  necessary  losses 


CONDITIONS    WHICH    PERMIT   EVAPORATION       197 

from  the  soil,  is  only  a  small  proportion  of  the  annual 
rainfall  of  most  of  the  cultivated  sections.  These  losses 
are  therefore  preventable;  and  that  this  is  true  is 
exemplified  -  by  the  large  difference  in  average  crop 
yield  on  those  lands  where  the  best  conservation  prac- 
tices are  in  vogue  over  those  where  they  are  neglected. 
It  should  be  remembered  that  over  the  vastly  larger 
proportion  of  cultivated  land  area  the  crop  yields  are 
controlled  more  directly  by  the  lack  of  water  than  by 
the  excess  of  water.  It  is  a  common  observation  that 
soils  which  ordinarily  give  a  low  yield  in  seasons  of 
normal  or  low  rainfall  give  good  yields  in  wet  season, 
indicating  how  large  a  dominating  factor  is  the  moisture 
supply.  For  the  moisture  concerns  not  only  its  direct 
use  as  a  food  and  carrier  for  the  plant,  but  by  its  influence 
on  solution,  and  other  essential  conditions  of  plant 
growth,  its  is  a  chief  dominating  factor  in  growth. 

Soil  evaporation  occurs  almost  entirely  at  the  surface. 
Exception  may  be  made  where  evaporation  occurs  into 
large,  deep  cracks  in  heavy  clay  soil,  which  is  the  primary 
source  of  subsoil  loss  in  such  cases.  If  this  be  prevented, 
as  it  may  be,  the  loss  will  be  very  small.  Since  evapora- 
tion is  chiefly  at  the  surface,  the  nearer  the  available 
store  of  moisture  is  held  to  the  surface,  the  larger  pro- 
portionate loss  will  occur.  This  principle  has  its  appli- 
cation in  the  amount  and  distribution  of  the  rainfall 
or  irrigation.  Frequent  small  rainfalls  are  much  less 
effective  than  less  frequent  rains  in  larger  amounts. 
For  if  the  rainfall  or  irrigation  produces  only  shallow 
percolation  before  the  water  assumes  capillary  forms, 
it  may  be  quickly  returned  to  the  surface,  and  lost. 


198        THE  PRINCIPLES    OF    SOIL    MANAGEMENT 

Also  there  is  a  certain  inherent  loss  in  the  most  careful 
field  practices,  which  are  proportionately  greater  with 
small  applications  of  water  than  with  large  ones.  It 
has  been  shown  (page  182)  that  as  the  capillary  films 
are  reduced  in  thickness  the  movement  becomes  in- 
creasingly difficult  and  slow.  Therefore  in  a  fine-tex- 
tured or  dense  soil,  where  evaporation  occurs  only  at 
the  surface,  the  top  layer  may  become  so  dry  in  warm, 
clear  weather  that  capillary  movement  practically 
ceases.  Therefore,  loss  is  also  stopped.  If  now  there 
comes  a  light  rainfall, — sufficient  to  replenish  the  super- 
ficial moisture  films,  but  not  enough  to  produce  deep 
percolation, — the  result  may  be  the  renewal  of  capillary 
movement,  which  will  ultimate  in  a  few  days  in  a  greater 
total  loss  than  would  have  occurred  had  there  been  no 
rainfall.  These  results  have  frequently  been  observed 
in  practice,  and  were  definitely  shown  in  field  moisture 
studies  made  by  Stewart.  In  moisture  studies  of  the 
soil  in  the  open,  and  under  a  muslin  shade  used  in  grow- 
ing wrapper  tobacco  in  the  Connecticut  valley,  it  was 
observed  that  a  small  rainfall  had  a  much  larger  effect 
on  the  soil-moisture  content  outside  than  inside  the  tent. 
A  rainfall  of  less  than  half  an  inch  increased  the  water 
in  the  surface  nine  inches  of  the  soil  outside  the  tent  to 
a  larger  extent  than  could  be  accounted  for  by  the  rain- 
fall. Careful  calculations  and  observatio'ns  indicated 
that  the  difference  represented  movement  up  from  the 
subsoil,  due  to  the  renewal  of  film  movement.  King 
has  obtained  similar  results  in  field  studies  which  he  has 
checked  experimentally.  This  emphasizes  the  desira- 
bility of  storing  water  as  deeply  in  the  soil  as  is  practi- 


RETENTION   OF   WATER   BY   MULCHES  199 

cable,  and  of  giving  a  few  relatively  large  applications 
rather  than  many  small  ones,  in  the  artificial  addition 
of  water. 

Surface  evaporation  may  be  reduced  in  two  ways: 
(1}  By  the  application  of  some  protective  covering 
to  the  moist  soil.  (2)  By  such  surface  treatment  as  will 
reduce  the  tendency  to  evaporation. 

95.  Mulches. — The  protective  covering  constitutes 
a  mulch.  That  is,  a  mulch  is  any  material  applied  to  the 


Fio.  62.  Two  types  of  soil  structure  .  On  the  right,  compact  soil,  due 
to  the  use  of  the  roller.  On  the  left,  the  same  soil,  loosened  at  the  surface  to 
form  a  mulch. 

surface  primarily  for  the  purpose  of  preventing  evapo- 
ration. It  may  at  the  same  time  fulfil  other  useful 
functions,  as  keeping  down  weeds  and  maintaining  a 
more  uniform  soil  temperature,  but  its  primary  use  is 
to  prevent  evaporation.  Of  course,  in  so  far  as  the 
growth  of  weeds  is  prevented,  moisture  loss  from  that 
source  is  eliminated,  and  at  the  same  time  plant  food 
is  conserved  for  the  regular  crop. 

Mulches    are    of    two    sorts:     (1)   Foreign    material 


200          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

applied  to  the  surface  of  the  soil.  (2)  Those  composed 
of  the  natural  soil  modified  by  appropriate  tillage. 

The  action  of  both  sorts  of  material  depends  on  the 
facts  shown  on  pages  180  and  189;  namely,  that  capillary 
action  may  be  changed  or  broken  by  sufficient  change 
in  the  texture  or  structural  properties  of  the  material, 
and,  second,  that  the  diffusion  of  water  vapor,  even  after 
evaporation  has  taken  place,  is  exceedingly  slow  through 
small  irregular  pore  spaces,  such  as  exist  in  all  materials 
effective  as  mulch.  Any  material  is  effective  as  a  mulch 
in  proportion  as  it  fulfils  these  conditions;  and  their 
practical  application,  therefore,  becomes  chiefly  a  matter 
of  selecting  that  material  which  meets  these  require- 
ments, and  may  be  readily  applied. 

Many  kinds  of  material  are  used  as  a  mulch.  Straw, 
chaff,  dead  weeds,  stubble,  leaves,  sawdust,  manure, 
boards,  canvas,  stone,  coarse  sand — all  of  these  are  used, 
and  many  other  waste  materials  which  may  be  available. 
They  act  as  a  cover  to  the  moist  soil,  so  that  water 
which  is  held  in  the  surface  of  the  soil,  or  is  brought  up 
by  capillarity,  must  evaporate  into  this  stagnant  and 
therefore  soon-saturated  atmosphere;  under  which  con- 
ditions the  loss  must  be  much  less  than  where  the  vapor 
is  freely  removed,  and  dry  air  brought  in  contact  with 
the  moist  soil.  All  of  these  materials  are  very  efficient 
as  a  mulch,  their  efficiency  depending  upon  their  thick- 
ness and  porosity.  Straw  and  leaves,  when  fresh  and 
dry,  will  reduce  evaporation  below  10  per  cent  of  the 
normal,  when  in  a  layer  three  or  four  inches  thick. 
As  they  decay  and  become  water-soaked  from  succes- 
sive rains,  their  efficiency  decreases;  but  they  retain 


ARTIFICIAL   MULCHES 


201 


an  efficiency  of  at  least  50  per  cent  for  a  long  period, 
or  until  they  are  so  decayed  that  they  acquire  decided 
capillary  capacity.  A  practice  based  upon  this  effect 
is  that  of  growing  potatoes  under  straw.  The  potatoes 
are  laid  upon  the  surface  of  the  ground,  and  covered 
deeply  with  straw,  which  keeps  the  surface  soil  so  moist 
that  the  potatoes  sprout  and  will  grow  a  reasonable 
crop  to  maturity,  when  the  straw  has  simply  to  be  raked 
back  and  the  tubers,  clean  and  smooth,  are  found  on  or 
very  near  the  surface.  Leaves,  including  pine  needles, 


Fio.  63.     A  very  stony  soil.    Boulders  and  gravel  serve  as  a  mulch,  promote 
drainage,  and  increase  the  warmth  of  the  soil. 


202         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

and  sawdust,  are  very  effective  as  a  mulch,  but  some 
precautions  should  be  observed  in  their  application. 
For  example,  the  oak  is  rich  in  tannic  acid,  which  may 
be  washed  out  of  the  mulch  into  the  soil  and  cause 
injury  to  its  producing  power,  by  its  effect  on  the  growing 
plant.  In  some  European  countries,  as  well  as  in  a  few 
places  in  America,  stone  has  been  drawn  on  the  soil, 
particularly  in  orchard  and  vineyard  culture,  to  serve 
as  a  mulch,  and  with  markedly  beneficial  effects.  Par- 
ticularly is  this  true  on  those  lands  too  steep  to  permit 
cultivation.  And,  as  a  corollary  to  this  practice,  it  has 
been  observed  in  the  fruit-growing  section  of  the  Ozark 
Mountains,  and  doubtless  in  other  regions,  that  the 
removal  of  stone  from  the  land  not  only  permits  the 
soil  to  become  more  hard,  but  also  reduces  crop  yield 
by  increasing  the  loss  of  moisture.  It  is  therefore  for 
the  farmer  to  decide  whether  the  inconvenience  to  tillage 
or  other  operations  due  to  the  presence  of  the  stone  may 
not  be  more  than  offset  by  their  beneficial  effects.  A 
layer  of  two  or  three  inches  of  coarse  sand  or  fine  gravel 
is  a  very  effective  mulch,  and  is  frequently  used  in  green- 
house practice. 

The  above-mentioned  mulch  materials  are  all  strictly 
artificial,  and  their  application  is  greatly  limited,  due 
to  the  lack  of  material  and  the  expense  involved.  They 
are  therefore  used  only  under  special  conditions.  But 
the  second  type  of  mulch  is  almost  universal  in  its 
practical  availability. 

Almost  any  soil  may  be  converted  into  an  effective 
mulch  by  proper  treatment.  This  treatment  will  differ 
with  the  character  and  condition  of  the  soil  and  the 


DUST   MULCHES  203 

climate.  Mulches  formed  from  the  natural  soil  are 
commonly  termed  "dust  mulches,"  or  more  expressively 
"dust  blankets."  A  dust  mulch  is  simply  an  air-dry 
layer  of  the  natural  soil  covering  the  moist  soil  below. 
It  may  be  in  a  compact  condition,  but  ordinarily  it  is 
loose  and  friable.  Its  creation  is  dependent  on  the  prin- 
ciples explained  on  pages  172  and  189  concerning  capil- 
lary movement  and  diffusion  of  water-vapor.  Under  arid 
conditions  where  the  atmosphere  is  dry  and  hot,  and  in 
free  circulation,  the  surface  soil  is  quickly  dried  out 
after  an  application  of  water.  This  drying  takes  place 
so  rapidly  that  the  capillary  films  quickly  become  so 
thin  that  movement  is  stopped,  and  no  more  water  is 
brought  to  the  surface.  The  soil  may  be  ever  so  hard 
and  compact,  but  so  long  as  it  is  kept  dry  it  very  effec- 
tively preserves  the  moisture  below.  The  more  rapid 
the  loss,  the  more  quickly  will  the  mulch  condition  be 
created,  and  therefore  the  less  the  total  loss  of  water 
is  likely  to  be.  This  has  been  demonstrated  by  Bucking- 
ham in  some  experiments  in  which  arid  climate  conditions 
were  created  at  the  surface  of  a  capillary  column 
forty-six  inches  in  height.  The  soil  was  a  fine  sandy 
loam,  the  equilibrium  distribution  of  water  in  which 
is  shown  in  the  curve  on  page  148.  At  first,  the  loss  under 
the  arid  conditions  was  very  rapid  and  exceeded  the 
humid  conditions,  but  the  rate  of  loss  soon  dropped 
considerably  below  the  humid  column,  and  continued 
to  fall  behind  during  the  twenty  days  of  the  experiment. 
This  experiment  was  conducted  under  the  most  difficult 
conditions  for  creating  a  mulch,  since  the  soil  used  was 
of  intermediate  fineness  and  had  a  large  effective  capil- 


204 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


lary  capacity,  and,  further,  it  had  a  full  supply  of  water 
at  the  bottom  of  the  column, — conditions  seldom  found 
in  practice,  and  certainly  not  common  under  arid- 
climate  conditions.  The  curves  of  water  loss,  showing 
the  mulching  effect  of  rapid  drying,  appear  below: 


6  10  16  20 

TIME   IN   DAYS 

FIG.  64.  Curves  showing  the  relative  evaporation  of  water  from  two  col- 
umns of  the  same  soil.  One  was  kept  in  a  dry  atmosphere  at  the  immediate 
surface.  The  other  was  maintained  under  normal  humid  climate  conditions  of 
moisture  and  temperature. 

For  the  reasons  presented,  the  moisture  supply  in 
arid  regions  appears  to  be  naturally  more  effectively 
conserved  than  in  humid  regions, — certainly  a  wise 
provision.  This  fact  is  to  be  connected  with  the  further 
one  that  capillary  movement  into  the  deep  subsoil  is 
very  slowr. 

The  mulcting  effect  described  above  gives  further 
emphasis  to  the  unwisdom  of  frequent  small  applications 
of  water  to  the  soil. 

In  humid  regions  the  natural  mulching  effect  is 
much  less  marked  than  in  arid  regions.  If  the  farmer 
would  produce  a  soil  mulch,  he  must  do  it  by  creating 
as  far  as  possible  the  arid  conditions.  That  is,  he  must 
bring  about  such  a  rapid  drying  of  the  surface  soil  as 
to  convert  it  into  a  mulch  which  will  retain  the  moisture 


MANAGEMENT    OF   MULCHES  205 

below.  Since  in  humid  regions  drying  is  usually  slow  and 
capillary  movement  strong,  the  process  is  hastened  by 
loosening  the  top  soil  by  frequently  stirring,  in  order 
(1)  to  hasten  the  drying  of  that  surface  portion  to  the 
point  where  capillarity  is  stopped,  and  (2)  to  reduce  its 
capillary  conductivity, — both  of  which  hasten  the  forma- 
tion of  the  mulch.  It  is  for  these  reasons  that  a  mulch 
is  generally  a  loose  layer  of  soil. 

The  management  of  the  mulch  is  evident  from  the 
principles  involved.  It  must  be  kept  dry  in  order  to  break 
up  capillarity.  In  humid  regions,  where  frequent  rains 
occur,  the  mulch  may  be  destroyed.  After  such  a  rain, 
when  the  soil  has  reached  the  proper  dryness,  it  should 
be  again  stirred,  to  renew  the  mulch.  On  heavy  clay 
soil  in  fine  tilth,  a  mulch  may  be  destroyed  by  very 
moist  foggy  weather,  or  by  a  number  of  days  of  very 
humid  atmosphere,  which,  by  condensation  of  moisture 
on  the  clay,  hastens  the  reestablishment  of  capillarity 
with  the  subsoil,  by  which  moisture  may  be  pumped 
up  and  lost.  This  is  to  be  overcome  by  occasional  stir- 
ring, as  conditions  may  require.  Another  important 
effect  of  the  mulch  on  clay  is  to  keep  the  shrinkage 
cracks  filled  up,  and  thereby  prevent  the  deep  drying- 
out  of  such  soil. 

When  perfectly  dry,  a  coarse  sand  and  a  pulverized 
clay  are  of  almost  the  same  practical  efficiency.  (See 
page  190.)  It  is  only  when  the  structure  becomes  that 
of  coarse  clods  or  stone  that  the  efficiency  is  greatly 
reduced.  A  cloddy  surface  soil  is  worse  than  a  smooth 
surface  with  no  mulch,  for  the  clods  are  free  to  evaporate 
water,  and  offer  small  protection  to  the  subsoil.  On 


206         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

the  other  hand,  the  pulverized  clay  has  so  great  hygro- 
scopic and  capillary  power  that  its  efficiency  as  a  mulch 
may  be  readily  destroyed  by  natural  climate  and  soil 
conditions  of  common  occurrence.  It  is  therefore  more 


Fio.  65.     An  example  of  clean,  thorough  tillage,  and  the  maintenance  of  an 
effective  "dust  mulch." 

difficult  to  maintain  a  dust  mulch  of  clay  than  of  sand. 
The  strong  natural  mulching  tendency  of  sand  may  be 
seen  on  sand-dunes,  where,  although  the  surface  is  dry 
and  hot,  moisture  may  be  exposed  by  the  toe  of  one's 
boot  at  any  season. 

A  perfectly  dry  dust  mulch  need  not  be  very  deep,  to 


DEPTH   OF  MULCHES  207 

be  effective.  One  inch  of  sand  will  permit  loss  by  diffu- 
sion of  less  than  three  inches  of  water  per  year,  under  the 
most  favorable  conditions.  In  practice,  however,  it  is 
found  that  two  or  three  inches  are  usually  most  effective 
because  of  capillary  action.  And  Fortier  has  concluded 
from  experiments  on  irrigated  soil  in  California  that  a 
ten-inch  mulch  conserves  more  moisture  than  one  of  less 
depth.  But  the  efficiency  of  the  ten-inch  mulch  as  com- 
pared with  the  four-inch  is  very  much  less  in  proportion 
to  depth,  and  the  latter  conserves  75  per  cent  of  the 
water  lost  where  no  mulch  was  used.  Sand  mulches  may 
be  thinner  than  clay  mulches.  King  found  in  Wisconsin 
that,  for  corn,  cultivation  with  a  small  toothed  culti- 
vator to  a  depth  of  three  inches  saved  more  moisture 
in  fifteen  cases  out  of  twenty  than  did  more  shallow 
tillage,  but  that  increase  in  depth  resulted  in  no  corre- 
sponding increase  in  efficiency.  The  sweep  or  blade  type 
of  cultivator  (Fig.  137)  may  be  used  more  shallow  than 
an  implement  producing  ridges.  The  mulch  should  be  no 
deeper  than  is  necessary  to  prevent  loss  of  water,  since 
this  top  layer  is  usually  most  rich  in  available  plant-food, 
particularly  nitrates,  and  the  roots  are  excluded  from 
it  by  tillage.  Unnecessary  depth  reduces  the  root  range. 
Some  results  from  an  experiment  conducted  at  Cornell 
University  serve  to  illustrate  the  relation  of  mulches 
and  weeds  to  soil  moisture  and  crop  production  in  a 
humid  region  in  a  season  of  good  rainfall.  The  crop 
grown  was  maize.  Every  third  plot  was  a  check,  and 
was  given  normal  treatment.  The  figures  show  the  in- 
crease or  decrease  in  5'ield  as  compared  with  the  nearest 
check  plots.  Moisture  determinations  were  made  on 


208 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


portions  of  the  plots  bearing  no  crop,  but  otherwise 
receiving  the  same  treatment  as  the  remainder  of  the 
plot.  The  table  thus  shows  the  moisture  conserved 
or  lost  by  treatment,  entirely  aside  from  that  transpired 
by  the  crop. 

TABLE   XXXVII 


Increased  (+) 
or 
decreased  (  —  ) 
yield 

Yields  calcu- 
lated to  basis 
of  100  on 
check  plots 

Soil 
moisture 
during 
August 

Compari- 
son soil 
moisture 
basis  of  100 
on  check 
plots 

Check  plot    

Pounds 

100 

Per  cent 
21.1 

100 

Weeds   removed,    but  not 
cultivated  

—157 

96 

18.2 

90 

Mulched  with  straw  

+  873 

121 

25.0 

130 

Check  plot    .... 

100 

18.2 

100 

No  cultivation;  weeds  al- 
lowed to  grow 

—2,888 

31 

9.8 

54 

One  cultivation;  weeds  al- 
lowed to  grow  

—109 

98 

17.0 

95 

Check  plot    

100 

17.7 

100 

The  application  of  the  dust  mulch  is  not  confined 
to  inter-tilled  crops  like  maize,  potatoes,  vineyards, 
fallow,  etc.  Under  some  conditions,  it  may  be  applied 
to  grain  fields  with  good  results.  In  those  sections  of 
the  country  where  "dry  farming"  is  practiced,  it  is  not 
uncommon  to  drag  the  grain  field  with  a  sharp-toothed 
harrow,  the  teeth  pointing  very  slightly  backward. 
This  is  begun  when  the  plants  are  small,  and  may  be  kept 
up  until  they  attain  a  considerable  size  or  until  they 
sufficiently  shade  the  ground  to  greatly  reduce  surface 
evaporation.  The  surface  soil  between  the  plants  is 
broken  up  and  converted  into  a  mulch.  Similar  to  this 


MULCHING   PLOW    LAND 


209 


is  the  use  of  the  harrow  in  the  early  stage  of  growth 
of  cultivated  crops,  by  which  the  weeds  are  kept  down 
and  a  mulch  created.  If  the  practice  is  begun  when  the 
plants  are  very  young — even  before  they  appear  above 
the  ground — so  that  the  formation  of  roots  very  near 
the  surface  is  prevented,  it  may  be  kept  up  to  very  a 
advanced  stage  of  growth  without  serious  injury. 


1- 
11 

10 

90 

.,, 

70 

,.,, 

M 
M 

•  ,. 

_A- 

/ 

\\ 

/ 

5 

X<<- 

f 

V 

i 

'^^ 

l\ 

/ 

A 

/! 

\  \ 
\ 

f  i 

\ 
\ 

i 

i 

\ 

\ 

i 

i 

\ 

i 

YIELD  OF  CROP 


.MOISTURE    IN  SOIL 


Fio.  66.  Curvi-s  representing  relative  yield  of  dry  matter  ami  mois'iire 
content  of  soil  on  field  plot*  given  different  cultural  treatments.  tS.-e  Table 
XXXVII.) 

But  dragging  only  after  the  plants  are  good-sized  may 
cause  serious  loss. 

96.  Mulching  plow  land. — -It  frequently  happens, 
especially  on  heavy  soil,  that  it  is  impracticable  to 
complete  plowing  before  the  soil,  if  left  in  its  natural 
condition,  becomes  too  dry  for  the  best  results.  In  such 
cases  it  is  frequently  practicable  to  quickly  form  some- 
thing of  a  mulch  by  use  of  the  disk  or  toothed  harrow. 
Further,  this  treatment  creates  numerous  lines  of  weak- 
ness which,  although  drying  may  progress  further 


210         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

than  is  desired,  will  cause  the  soil  to  break  up  into  a 
much  better  condition  than  if  the  surface  had  not  been 
treated.  The  width  of  the  disk  or  harrow  makes  it 
possible  to  cover  a  large  area  in  a  short  time,  and 
thereby  considerably  lengthen  the  period  during  which 
plowing  can  be  satisfactorily  done,  as  well  as  conserving 
moisture  for  the  succeeding  crops. 

To  summarize  briefly  the  cardinal  points  in  mulch 
control:  (1)  They  are  more  effective  and  more  easily 
maintained  in  an  arid  than  in  a  humid  climate.  (2)  Their 
efficiency  depends  directly  on  their  dryness  and  fineness. 
(3)  Sandy  soil  is  more  easily  maintained  as  mulch 
than  clay  soil.  (4)  From  two  to  three  inches  is  ordi- 
narily the  most  effective  depth.  (5)  After  heavy  rain, 
the  soil  mulch  must  be  renewed  by  tillage,  and  this  is 
much  more  urgent  on  clay  than  on  sand  soil.  Even 
without  rain,  a  clay  mulch  may  become  inefficient. 
(6)  Tillage  for  mulch  purposes  must  ordinarily  be  more 
frequent  in  the  spring,  or  humid  season,  than  at  other 
times  of  the  year.  (7)  The  use  of  foreign  materials 
as  mulch  may  be  justified  under  special  circumstances. 

97.  Fall  and  spring  plowing. — Fall  and  early  spring 
plowing  owe  much  of  their  efficiency  to  the  conservation 
of  moisture  effected  through  the  creation  of  a  mulch 
over  the  surface.  Fall  plowing  may  be  practiced  for 
a  number  of  reasons,  but  in  regions  of  deficient  rainfall, 
particularly  in  the  winter,  the  conservation  of  the  mois- 
ture in  the  soil  at  the  close  of  the  growing  season  is  an 
important  consideration.  This  practice  is  well  adapted 
to  those  soils  in  the  semi-arid  section  that  do  not  blow 
too  badly  when  fall-plowed,  and  where  the  winter  rain 


FALL  AND  SPRING  PLOWING  211 

is  not  sufficient  to  saturate  the  soil.  If  the  soil  is  left 
in  the  bare,  hard  condition  resulting  from  the  removal 
of  a  crop  of  maize,  wheat  or  barley,  a  large  amount  of 
water  may  be  lost  by  evaporation  during  the  fall  months. 

For  the  average  farmer  in  humid  regions  where  the 
winter  rainfall  is  sufficient  to  saturate  the  soil,  early 
spring  plowing,  coupled  with  tillage,  is  much  more 
important.  Not  only  may  moisture  be  conserved,  but 
the  soil  is  worked  at  the  stage  when  it  yields  most 
readily  to  pulverization.  Fallow  land,  and  bare  stubble 
land  of  fine-textured  soil,  are  most  benefited,  since  they 
become  compact  to  the  very  surface  as  a  result  of  the 
winter  rain  and  snow,  and  are  therefore  in  condition 
for  the  most  rapid  loss  of  water.  They  should  be  plowed 
as  early  as  practicable  without  injury  to  their  structure. 
At  the  Wisconsin  station,  two  adjacent  pieces  of  land 
very  uniform  in  character  were  plowed  seven  days  apart. 
At  the  time  the  second  plot  was  plowed,  it  was  found  to 
have  lost  1.75  inches  of  water  from  the  surface  four  feet 
in  the  previous  seven  days;  while  the  earlier  plowed 
piece  had  actually  gained,  doubtless  by  increased  capil- 
larity, a  slight  amount  of  water  over  that  it  contained 
when  plowed.  There  was  a  gain  of  nearly  two  inches 
of  water  in  the  root  zone  as  a  result  of  plowing  one  week 
earlier,  enough  to  produce  1,500  pounds  of  dry  matter 
in  maize  per  acre,  if  properly  conserved. 

In  arid  and  semi-arid  regions,  and  in  other  sections 
where  heavy  soil  is  plowed  in  the  late  summer,  and 
especially  where  a  large  crop  of  green  manure  or  a  large 
application  of  coarse  strawy  manure  is  plowed  under 
at  any  season,  it  is  essential  that  the  lower  part  of  the 


212          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

furrow  slice  be  brought  into  close  contact  with  the 
subsoil  as  soon  as  possible,  in  order,  (1)  that  the  best 
possible  capillary  contact  with  the  subsoil  may  be  estab- 
lished; (2)  that  there  may  be  sufficient  moisture  to 
promote  the  rapid  decay  of  the  organic  matter;  (3)  to 
increase  the  moisture  capacity  and  cut  down  loss  by 
percolation  and  evaporation.  This  may  be  accomplished 


FIG.  67.     The  Campbell  subsurface  packer 

by  rolling  the  plowed  land,  but  in  particularly  dry 
regions  the  practice  of  sub-surface  packing  is  advan- 
tageous. The  aim  of  sub-surface  packing  is  to  pack  the 
soil  and  still  leave  a  loose  mulch  on  the  surface.  The  sub- 
surface packing  may  very  well  be  applied  to  land  sub- 
soiled  in  the  spring.  Land  subsoiled  in  the  fall  will  not, 
as  a  rule,  require  this  treatment, — certainly  not  in  the. 
humid  sections  of  the  country.  To  accomplish  sub- 
surface packing,  a  special  group  of  implements  have 
been  devised,  one  of  which  consists  of  small  wheels 


SUB-SURFACE   PACKING  213 

placed  five  inches  apart  on  an  axle.  The  rim  is  much 
thickened  and  is  triangular  in  shape,  with  the  thin 
edge  outward,  so  that  the  effect  is  to  give  a  decided 
downward  and  sidewise  pressure,  while  enough  fine 
earth  is  left  at  the  immediate  surface  to  serve  as  a  mulch. 
98.  Other  surface  treatments. — Other  surface  treat- 
ments aim  to  decrease  the  tendency  to  evaporation. 
When  evaporation  takes  place  into  a  quiet  atmosphere, 
the  layer  next  to  the  soil  soon  becomes  so  nearly  satu- 
rated with  moisture  that  the  rate  of  evaporation  is 
greatly  reduced.  But  if  the  atmosphere  is  in  free  circu- 
lation,— that  is  if  there  is  wind, — the  saturated  air  is 
removed,  and  more  dry  air  is  brought  over  the  soil  into 
which  evaporation  is  continuous.  The  drying  effect  of 
wind  is  very  generally  recognized.  Warm  winds  in 
spring  and  early  summer  are  recognized  as  particularly 
drying,  and  in  the  semi-arid  section  just  east  of  the 
Rocky  mountains  so-called  hot  winds  sometimes  do 
great  damage  to  growing  crops  by  the  rapid  evaporation 
they  produce.  Obviously  anything  which  reduces  the  free 
circulation  of  the  air — "breaks  the  wind" — will  reduce 
evaporation.  In  practice,  this  takes  the  form  of  wind- 
breaks of  various  types.  Strips  of  timber  are  commonly 
grown  or  retained  for  this  purpose.  Wooden  fences 
and  walls  of  one  sort  or  another  have  a  similar  effect. 
Wind-breaks  composed  of  growing  plants  have  the 
disadvantage  that  for  a  considerable  distance  beyond 
the  spread  of  their  branches  their  roots  penetrate  the 
soil  and  use  the  moisture,  which  is  one  reason  for  the 
smaller  growth  of  crops  near  trees.  But  artificial  shelters 
do  not  have  this  advantage.  Bearing  on  the  efficiency  of 


214         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

wind-breaks,  results  by  King  show  that  when  the  rate 
of  evaporation  at  20,  40  and  60  feet  to  the  leeward 
of  a  black  oak  grove  15  to  20  feet  high  was  11.5  cc.,  11.6 
cc.,  and  11.9  cc.,  respectively  from  a  wet  surface  of  27 
square  inches  the  evaporation  was  14.5,  14.2  and  14.7 
cc.,  at  280r  300  and  320  feet  distant,— or  24  per  cent 
greater  at  the  outer  stations  than  at  the  inner  ones.  A 
scanty  hedge-row  reduced  evaporation  30  per  cent  at 
20  feet,  and  7  per  cent  at  150  feet,  below  the  evapora- 
tion at  300  feet  from  the  hedge. 

On  sandy  soil,  wind-breaks  prevent  the  blowing  of 
the  dry  surface  soil,  which  would  expose  a  fresh  surface 
of  wet  soil  from  which  evaporation  would  be  increased. 

The  glass  house  reduces  evaporation  by  preventing 
winds.  Some  crops  are  grown  only  in  the  shade  of  other 
crops,  where  they  are  not  only  protected  from  the  sun 
but  from  evaporation  by  the  stagnating  effect  of  the 
surrounding  vegetation  on  the  atmosphere.  Grass 'pro- 
tects the  surface  of  the  soil  from  evaporation,  acting 
like  a  mulch.  The  largest  application  of  this  principle 
is  in  the  tents  used  in  growing  wrapper  tobacco  in  Florida 
and  the  Connecticut  valley,  and,  to  a  less  extent,  for 
other  special  crops  in  various  parts  of  the  country.  The 
most  common  form  of  the  tent  is  a  frame  eight  or  nine 
feet  high,  over  which  is  spread  a  loosely  woven  cloth- 
cheese-cloth.  Investigations  by  Stewart  in  Connecticut 
showed:  (1)  That  the  tent  greatly  reduced  the  velocity 
of  the  wind.  This  reduction  amounted  to  93  per  cent 
when  the  outside  velocity  was  seven  miles  per  hour, 
and  85  per  cent  when  the  outside  velocity  was  twenty 
miles  per. hour,  there  being  a  small  regular  decrease  in 


WINDBREAKS  AND  TENTS  TO  SAVE  WATER       215 

relative  efficiency  with  increased  velocity  of  the  wind. 
(2)  The  relative  humidity  under  the  tent  was  higher 
than  outside,  and  during  a  good  part  of  the  time 
attained  a  difference  of  10  per  cent.  The  effect  of  this 
was  to  reduce  evaporation,  by  from  53  to  63  per  cent 
on  different  days  in  July,  in  spite  of  a  higher  tempera- 
ture inside  the  tent.  (3)  The  direct  effect  of  these  was 
to  increase  the  moisture  content  in  the  soil  in  spite  of  a 
larger  crop  growth  under  the  tent.  These  differences 
are  shown  by  the  following  curves,  which  represent  the 
per  cent  of  water  in  the  soil  to  a  depth  of  nine  inches 
from  June  13  to  August  1. 


PERCENT.  OF  WATER 
M  t*  1 
O  w  o  ex 

^  -•/ 

\-* 

/  \ 

i 

^"V*" 

•Os,---*  INSIDE 

-XA 

r"^  

__l                ^v./ 

\ 

\J  \ 

"\ 

A 

JUNE    I3TM                                                                   DATE                                                                    AUGUST    1ST 
MM    .10-1.64-.17                      .22-.03                          .17    M                           .24             .2-.16-.10 
INCHES  RAIN 

Fio.  68.  Curves  representing  the  per  cent  of  moisture  in  a  sandy  soil  to  a 
depth  of  nine  inches  inside  and  outside  of  a  loosely  woven  cloth  tent,  July  13 
to  August  1,  Western  Connecticut. 

Not  only  was  the  effect  of  the  tent  to  prevent  evapo- 
ration and  thereby  increase  the  average  moisture  con- 
tent of  the  soil,  but  the  soil  was  able  to  maintain  a  more 
uniform  content,  due  to  the  more  free  movement  and 
adjustment  of  the  capillary  water  under  the  tent — con- 
ditions most  conducive  to  rapid  crop  growth.  (See 
page  172.) 


216          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

The  velocity  of  the  wind  next  to  the  ground  may  be 
checked  by  ridging  the  soil.  It  is  doubtful  if  this  prac- 
tice conserves  moisture,  because  more  surface  is  exposed 
over  which  evaporation  may  take  place.  On  the  other 
hand,  wide  experience,  as  well  as  investigation,  indicates 
that  for  the  conservation  of  water  level  culture  is  better 
than  ridged  culture.  This  principle  has  led  to  the  gradual 
abandonment  of  the  practice  of  "laying  by"  corn  and 
potatoes  with  a  high  ridge.  In  all  regions  of  deficient 
rainfall,  the  best  practice  prescribes  level  tillage  and 
a  fine,  dry  mulch,  both  of  which  are  attained  by  the 
frequent  use  of  shallow-running  small-toothed  culti- 
vators. Many  experiments  have  demonstrated  the 
larger  crop  yields  to  be  obtained  from  this  practice, 
on  the  average. 

The  removal  of  weeds  has  been  mentioned  as  a  means 
of  conserving  moisture.  The  plants  serve  to  expand 
evaporating  surface  in  the  same  way  as  ridged  culture. 
(See  page  195.) 

99.  Increasing  the  water  capacity. — Increasing  the 
water  capacity  of  the  soil  may  be  effective  in  conserving 
soil  moisture  by  holding  more  of  the  water  which  falls. 
The  first  aim  should  be  to  get  the  rainfall  or  irrigation 
water  into  the  soil.  It  is  well  known  that  after  a  long 
dry  period  when  the  soil — particularly  a  fine-textured 
soil — has  become  dry  and  hard,  the  first  rainfall  may 
be  largely  lost  by  running  away  over  the  surface.  Sudden 
showers  are  almost  entirely  lost  in  this  way,  because 
not  only  is  the  water  repelled,  but  the  small  amount  which 
is  absorbed  is  held  so  near  the  surface  that  it  is  quickly 
lost.  Gentle  rains  are  usually  much  more  effective 


INCREASING    WATER   CAPACITY  217 

than  sudden  showers  in  soaking  up  the  soil.  On  the 
other  hand,  if  the  soil  is  loose  and  porous,  all  the  water 
which  is  applied  sinks  into  the  soil  and  may  percolate 
deeply.  It  is  this  condition  which  should  be  maintained. 
Correlated  with  the  loose  surface  soil  is  the  rough  surface 
maintained  in  level  sections  of  strong  winds,  where 
a  considerable  part  of  the  precipitation  falls  as  snow. 
A  rough  surface  holds  the  snow  against  blowing,  and 
upon  melting  in  the  spring  it  enters  the  soil. 

The  moisture  taken  up  by  the  soil  should  be  retained 
and  conserved  by  appropriate  cultivation.  It  will  be 
apparent  from  the  principles  which  have  been  outlined 
that  all  soils  may  not  be  managed  in  the  same  way, 
to  increase  their  moisture  capacity.  In  some  the  end 
is  accomplished  by  loosening  the  structure,  and  in  others 
by  compacting  the  structure.  Cultivation,  the  roller, 
the  subsoil  plow,  or  fall  plowing,  are  to  be  adopted 
in  so  far  as  they  accomplish  the  desired  result  on  the 
particular  soil  in  hand.  The  opposite  effect  of  the  same 
treatment  on  different  soils  is  shown  by  the  following 
figures. 

TABLE  XXXVIII 


Amount  of 

Per  rent  of 

Soil 

Condition 

water 

water 

taken  up 

taken  up 

Clay  loam          

{ 

Loose  

178 

43.6 

\ 

f  ompact    .  . 

85 

23.8 

f 

Loose  

182 

44.1 

Silt  loam                .  .  . 

.  .  \ 

\ 

Compact    .  . 

90 

15.3 

( 

Loose.  . 

158 

26.8 

Medium  sand  .  . 

j 

'  \ 

Compact    .  . 

147 

23.2 

218 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


The  proportionate  increase  in  the  water  capacity 
of  the  sand  and  decrease  of  the  clay  loam  is  here  well 
shown,  and  doubtless,  if  the  column  had  been  longer, 
the  compact  sand  would  have  had  a  greater  absolute 

capacity  than  when 
loose. 

Deep  plowing  is 
greatly  to  be  re- 
commended as  a 
practice  to  increase 
the  moisture  capa- 
city of  the  soil, 

Fio.  69.     Subsoiler  that  loosens  the   subsoil  by      particularly    where 
raising  and  breaking  it.  Organic     matter     is 

well  supplied.  It  creates  a  deep  soil,  and  should  estab- 
lish the  best  conditions  for  the  storage  of  moisture,  as 
well  as  food  for  the  plant.  If  organic  matter  is  not 
supplied,  deep  plowing  is  not  advisable  on  light  sandy 
soil;  but  on  clay  soil  it  is  beneficial  because  of  the 
loosening  or  granulating  effect. 

The  practice  of  subsoiling  aims  to  loosen  up  the  struc- 
ture of  the  deep  subsoil  without  turning  the  material 
to  the  surface.  It  increases  the  ease  of  root  penetration, 
the  rate  and  depth  of  percolation,  and  on  clay  soil  it 
increases  the  water  capacity.  Subsoiling  is  unnecessary 
and  may  even  be  injurious  on  sandy  soil,  and  on  clay 
soils  must  be  used  with  discretion.  It  is  difficult  to 
secure  the  proper  moisture  condition  of  clay  subsoils 
for  plowing  in  the  spring  in  time  for  spring  planting. 
The  soil  may  be  in  good  working  condition,  or  even  dry, 
while  the  subsoil  is  wet  enough  to  puddle.  On  the  other 


SUBSOILING   FOR   MOISTURE   CONTROL 


219 


hand,  if  the  subsoil  gets  dry  enough  to  break  up,  it  may 
remain  so  loose  and  lumpy  during  the  remainder  of 
the  season  that  capillarity  is  largely  destroyed,  and 
crops  suffer  from  shallow  rooting  and  lack  of  moisture. 
Decrease  in  crop  yields  as  a  result  of  subsoiling  in  spring 
are  frequently  reported.  On  the  other  hand,  subsoiling 
in  the  fall,  although  usually  more  difficult  to  accomplish, 
is  more  likely  to  result  in  benefit.  The  cloddy  condition 
which  may  be  developed  is  largely  broken  down  in  re- 
gions of  heavy  winter  rain  by  the  saturated  condition. 
Still  the  structure  does  not  become  nearly  so  compact 
as  before  the  treatment,  and  good  results. 

King  presents  figures  which  show  that,  as  a  result 
of  the  application  of  1.34  inches  of  water,  the  soil  which 
had  been  subsoiled  to  a  depth  of  twenty-one  inches 
retained,  after  a  period  of  four  days,  65.6  per  cent  more 
water  in  the  surface  four  feet  than  the  adjacent  land  not 
subsoiled. 

Not  only  is  sub- 
soiling  effective  to 
increase  the  abso- 
lute water  capa- 
city, but  it  may 
strengthen  the 
capillary  or  film 
movement  to  such 
an  extent  that  an 
important  amount  of  water  is  drawn  up  from  the  deeper 
subsoil  or  from  adjacent  zones  not  so  treated.  A 
"hardpan"  layer  below  the  plow  depth  may  seriously 
interfere  with  the  upward  movement  of  water  and  the 


FIG.    70. 


Subsoiler  that  loosens   the   subsoil  by 
breakinR    through. 


220 


THE   PRINCIPLES    OF   SOIL   MANAGEMENT 


penetration   of  roots.    This   condition    may   be   largely 
corrected  by  subsoiling. 

Coupled  with  deep  plowing  and  subsoiling,  subsur- 
face packing  is  often  very  beneficial.  Particularly  is 
this  true  in  early  fall  and  late  spring  plowing,  where 
the  soil  is  likely  to  be  cloddy  and  to  make  poor  capillary 
contact  with  the  subsoil.  Spring  crops  may  be  greatly 
injured  by  this  condition.  The  subsurface  packer  crushes 
the  clods,  presses  the  furrow  slice  down  more  firmly 
on  the  subsoil  without  compacting  the  surface  soil. 


Fio.  71.     "Clod  crusher"  and  sub-surface  packer. 

It  leaves  a  light  mulch  on  the  top  to  hold  moisture. 
Not  only  is  it  useful  in  improving  the  soil  structure 
under  the  conditions  just  mentioned,  but  it  promotes 
the  decay  of  organic  manures  and  assists  plant  roots 
in  penetrating  into  the  subsoil  below,  where  they  may 
have  a  larger  moisture  and  food  supply. 

Increase  in  the  humus  content  stands  next  to  modifi- 
cation in  texture  and  structure  as  a  means  of  increasing 
the  water  capacity  of  the  soil  in  accordance  with  the  prin- 
ciples explained  on  pages  144  and  153.  It  accomplishes 


IRRIGATION  221 

this  not  only  through  its  own  large  water  capacity, 
but  by  its  favorable  influence  on  the  structure  of  the 
soil.  It  should  be  worked  deeply  into  the  soil,  in  order 
that  its  many  beneficial  effects  may  be  brought  to  bear 
on  as  large  a  volume  as  possible.  It  is  especially  favored 
as  the  adjunct  of  deep  plowing  and  the  use  of  lime  for 
improving  soil  condition,  particularly  clay  soil. 

The  means  for  increasing  the  organic  content  of  the 
soil  have  been  discussed.  (See  page  131.)  They  include 
the  application  of  animal  manures  and  other  refuse, 
and  the  growth  of  crops  for  green  manure,  together  with 
that  crop  rotation  which  promotes  the  accumulation 
of  crop  remains,  and  that  type  of  farming  which  removes 
the  smallest  proportion  of  the  crop  from  the  farm  and 
returns  the  largest  proportion  to  the  soil. 

100.  Irrigation.— Irrigation  is  the  third  method  by 
which  the  soil  moisture  may  be  increased.  It  is  the  prac- 
tice of  directly  adding  water  to  the  soil,  to  supplement 
the  natural  rainfall.  It  is  chiefly  identified  with  the 
arid  and  semi-arid  sections  of  the  country,  where  the 
annual  rainfall  is  small.  It  is  customary  to  consider  a 
region  as  having  a  semi-arid  climate  when  the  rainfall 
is  between  ten  and  twenty  inches,  and  arid  when  it  is 
less  than  ten  inches.  These  limits  are  arbitrary  and 
necessarily  elastic,  because  the  actual  aridity  of  a  region 
depends  on  other  factors  than  the  total  annual  rainfall. 
It  depends  on  the  distribution  of  the  rainfall,  the  climate, 
particularly  temperature,  and  the  character  of  the  soil. 

While  irrigation  has  been  chiefly  identified  with  arid 
and  semi-arid  sections  (see  map,  page  137),  it  is  not 
limited  to  those  regions,  and  is  applicable  under  any 


222         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

condition  where  the  natural  rainfall  is  deficient  at  any 
period  of  the  growing  season.  Consequently,  irrigation 
is  practised  even  under  the  very  humid  climate  of 
Florida,  with  sixty  inches  of  rainfall,  around  New 
York  City  and  Boston,  with  forty  inches  of  rainfall,  and 
at  many  other  places  in  the  United  States  and  Europe, 
where  a  so-called  humid  climate  prevails.  In  these  latter 
places  it  is  identified  with  special  crops  of  high  value 
which  will  justify  the  expense  involved.  In  France,  Ger- 
many and  other  European  countries,  there  are  extensive 
areas  of  grass  land  which  are  artificially  watered,  often 
with  sewage,  which  adds  the  element  of  food  supply  as 
well  as  water.  Of  course,  all  greenhouse  management  in- 
volves the  practice  of  irrigation. 

Many  engineering  problems  are  involved  in  the  prac- 
tice of  irrigation,  and  have  to  do  with  the  collection, 
storage  and  application  of  water  to  the  land.  But  the 
principles  which  govern  the  application — the  method, 
time  and  amounts  of  water — suitable  for  each  crop  and 
soil  are  purely  agricultural  considerations,  to  be  han- 
dled in  each  case  as  the  local  conditions  may  indicate. 

The  amount  of  water  necessary  to  be  added  to  pro- 
duce a  full  crop  constitutes  the  "duty,"  or  efficiency, 
of  water.  It  is  the  least  amount  of  water  which  will 
produce  a  given  yield  under  a  given  set  of  conditions. 
The  "duty  of  water"  depends  upon  a  great  many 
factors;  in  fact,  is  limited  by  as  many  things  as  affect 
the  moisture  supply  of  soils  in  humid  regions.  The  dis- 
cussion of  irrigation  which  follows  presupposes  an  ade- 
quate supply  of  water,  a  condition  often  not  fulfilled. 
For  example,  the  area  of  the  Western  States  containing 


CONDITIONS  REQUIRING   IRRIGATION 


223 


public  lands  is  973  million  acres,  of  which  Newell  esti- 
mates that  about  70,000,000  is  of  a  desert  character. 
At  the  present  time,  irrigation  is  practiced  on  less  than 
1  per  cent  of  this  area,  and  the  total  water  supply  is 
estimated  to  be  sufficient  for  less  than  10  per  cent  of  the 
total  area. 


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FIQ.  72.  Map  of  the  western  portion  of  the  United  States,  showing  in  black 
the  irrigated  land,  and  in  dots  the  area  which  may  be  irrigated  if  all  the  avail- 
able water  supply  is  utilized  . 


224          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

101.  Factors  affecting  the  duty  of  water. — Eleven 
factors,  as  follows,  affect  the  duty  of  water  in  irriga- 
tion: 

(1)  The    peculiarities    of    the    crop  (see  page  134). 
Some  crops  require  much  more  water  than  others  for 
their    growth    and    maturity.     Even    certain    varieties 
may  require  much  more  water  than  others  of  the  same 
species. 

(2)  The  physical  character  of  the  soil.   If  the  applica- 
tion of  water  is  such  that   leaching  may  take  place, 
more  water  will  be  lost  through  sand  than  through  clay. 
The  character  of  the  soil  also  determines  the  effective- 
ness of  the  mulch,  which  may  be  maintained. 

(3)  The  character  of  the  subsoil. 

(4)  The  frequency  of  irrigation. 

(5)  Amount  and  distribution  of  the  rainfall.    These 
last  two  factors  are  closely  related  in  their  effect  on  the 
duty  of  irrigation  water.     Their  frequency  determines 
the  proportion  of  the  water  which  will  be  lost  by  surface 
evaporation.     (See  page  198.) 

(6)  The  amount  and  time  of  applying  water.    Water 
applied  in  the  evening  will  be  more  efficient  than  when 
applied  in  the  morning,  because  during  the  cool  night 
it  will  have  opportunity  to  diffuse  deeply  into  the  soil, 
where  the  hot  sun  of  the  following  day  will  have  less 
effect  upon  it  than  if  the  water  were  applied  in  the 
morning. 

(7)  The   climate.     Other   things   being   equal,    more 
water  will  be  required  in  a  warm,  windy  climate  than  in 
one  of  a  cool,  quiet  atmosphere.    This  factor,  of  course, 
largely  determines  the  rate  of  evaporation. 


AMOUNT  OF  WATER  USED  IN  IRRIGATION          225 

(8)  Method  of  applying  water.    The  furrow  system 
is  usually  more  economical  of  water  than  the  flooding 
system,  because  less  opportunity  is  given  for  evapora- 
tion. 

(9)  The  fertility  of  the  land,  as  distinguished  from  its 
physical    properties,    determines    the    duty    of    water 
through  its  influence  on  the  size  of  crop  which  may  be 
produced.    A  large  crop  is  more  economical  of  water 
than  a  small  one,  but  a  large  crop  will  require  a  larger 
total  amount  of  water. 

(10)  The   closeness   of   planting   affects   the   loss   of 
water  in  much  the  same  way  as  a  large  or  a  small  crop: 
(a)  By  determining  the  total  amount  of  water  which 
must  be  used  directly  by  the  plants;  and  (b)  by  shading 
the  ground  and  cutting  down  temperature  and   wind 
movement  more  or  less,  it  decreases  the  loss  of  water 
directly  from  the  soil. 

(11)  The    tillage    practice    affects    the    efficiency    of 
water  under  irrigation  as  it  does  the  efficiency  of  rainfall 
in  humid  regions.    If  lax  conservation  methods  are  used, 
much  more  water  will  be  needed  than  where  the  best 
tillage  processes  are  applied. 

For  these  reasons,  it  is  not  possible  to  specify  any 
definite  amount  of  water  which  should  be  used  in  the 
practice  of  irrigation.  It  varies  widely  for  different  sec- 
tions of  the  world  and,  since  it  is  very  common  to  meas- 
ure the  total  amount  of  water  supplied  at  the  head  of 
the  intake  canal.it  is  largely  determined  by  seepage  from 
the  canals  and  ditches.  (See  page  134.)  The  amounts 
of  water  which  are  applied  in  different  irrigation  sec- 
tions are  given  by  different  authorities  as  follows: 


226         THE   PRINCIPLES  OF  SOIL   MANAGEMENT 
TABLE  XXXIX 


Acres  irrigated 
per  second-foot 
of  water  used 

Equivalent  to 
inches 
per  ten  days 

Northern  India  

60-150 

3.96-1.580 

Italy  

65-  70 

3.66-3.400 

Idaho         

60-  80 

3.97-2.980 

Utah                           

60-120 

3.97-1.980 

San  Joaquin  Valley,  California  .  .  . 
Santa  Clara  Valley,  California  .  .  . 

100-150 
150-300 

2.38-1.580 
1.58-  .798 

In  Sefi,  on  the  lower  Nile  canals  in  Egypt,  one  second- 
foot  is  said  to  be  sufficient  for  350  acres,  as  managed. 
In  the  humid  regions  much  less  water  need  be  added 
by  irrigation,  and  is  necessary  only  to  supplement  the 
rainfall  in  the  drought  periods — to  fill  in  the  gaps.  Ordi- 
narily only  a  few  inches  per  season  are  needed,  usually 
toward  the  latter  part.  Dr.  Voorhees  has  compiled  the 
following  figures,  which  show  the  percentage  of  years 
in  which  there  was  a  deficiency  of  one  inch  or  more  per 
month  in  the  rainfall,  as  compared  with  the  average. 

TABLE  XL 


One  month 

Two  months 

Three  months 

New  York,  1836-1895  .... 
Philadelphia,  1868-1895  .  . 

Per  cent 
75 

88 

Per  cent 

42 
56 

Per  cent 
21 

30 

In  this  region,  the  deficiency  is  most  likely  to  occur 
in  the  summer  season.  The  records  show  that  during 
one-fourth  of  the  term  there  is  a  deficiency  of  rainfall 
covering  three  months.  Considering  the  monthly  rain- 


UNITS  OF   WATER   MEASUREMENT  227 

fall  to  be  from  two  to  three  inches,  a  deficiency  of  one 
inch  amounts  to  from  one-half  to  one-third  of  the  total, 
which  must  be  a  serious  hindrance  to  crop  growth, 
without  the  most  careful  soil  management. 

On  light,  sandy  soils,  and  with  careless  tillage  in 
general,  the  above  figures  indicate  that  there  may  fre- 
quently be  occasion  for  irrigation.  The  annual  rainfall 


FIG.  73.    Flume  for  measuring  miner's  inches. 

is   ample   for  full    crop  production,  if   it  could  all   be 
utilized. 

Many  units  are  used  in  the  measurement  of  water 
for  irrigation.  The  two  most  common  methods  of  stating 
the  quantity  of  water  used  are:  (1)  In  depth  of  water 
over  the  area,  as  acre-inches  or  acre-feet.  (2)  A  given- 
sized  stream  flowing  through  the  growing  season.  The 
two  most  common  units  under  the  latter  system  are  the 
second-foot  and  the  miner's  inch.  It  is  frequently  esti- 
mated that  a  flow  of  a  second-foot  of  water — one  cubic 


228        THE  PRINCIPLES    OF    SOIL    MANAGEMENT 

foot  per  second — through  a  growing  season  of  ninety 
days,  is  sufficient  to  irrigate  one  hundred  acres.  This 
is  sufficient  to  cover  the  area  21.3  inches  deep,  and  is 
equivalent  to  a  little  over  seven  inches  per  month. 
(See  pages  135  and  137.) 

The  miner's  inch  varies  in  value  in  different  sections. 
It  is  most  commonly  denned  as  the  amount  of  water 
which  will  flow  from  an  opening  one  inch  square  under 
a  pressure-head  of  six  inches  above  the  top  of  the  orifice, 
during  a  year,  and  is  considered  sufficient  to  irrigate 
from  5  to  10  acres.  It  is  equivalent  to  about  1.5  cubic 
feet  per  minute,  or  21.6  inches  over  10  acres  in  a  season, 
which,  it  will  be  observed,  is  practically  the  same  appli- 
cation as  one  second-foot,  as  stated  above. 

102.  Methods  of  applying  water. — In  his  book  on 
Irrigation  and  Drainage,  King  makes  the  following 
cogent  statement  with  reference  to  the  application  of 
water  in  irrigation  practice.  "When  water  has  been 
provided  for  irrigation,  and  brought  to  the  field,  where 
it  is  to  be  applied,  the  steps  which  still  remain  to  be 
taken  are  far  the  most  important  in  the  whole  enter- 
prise,— not  excepting  those  of  engineering,  however 
great, — which  may  have  been  necessary  in  providing 
a  water-supply  that  shall  be  constant,  ample  and  moder- 
ate in  cost;  for  failure  in  the  application  of  water  to  the 
crop  means  utter  ruin  for  all  that  has  gone  before." 

"To  handle  water  on  a  given  field  so  that  it  shall  be 
applied  at  the  right  time,  in  the  right  amount,  without 
injuring  the  crop,  requires  an  intimate  acquaintance 
with  the  conditions,  good  judgment,  close  observation, 
skillful  manipulation,  and  patience  after  the  field  has 


METHODS  OF  APPLYING  WATER        229 

been  put  into  excellent  shape;  and  just  here  is  where  a 
thorough  understanding  of  the  principles  governing 
the  wetting,  puddling  and  washing  of  soils,  and  possible 
injury  to  the  crop  as  a  result  of  irrigation,  becomes  a 
matter  of  the  greatest  moment."  (See  page  103  et  seq.) 

Mead  reports  that  there  are  over  thirty  methods  of 
distributing  water  in  use  in  the  United  States.  Each 
of  these  has  its  special  adaptations  as  to  soil,  crop, 
water  supply,  climate  and  land  contour.  All  of  these 
methods  may  be  grouped  under  four  general  heads,  the 
further  differences  being  in  detail  of  application  and 
not  in  essential  principles. 

These  are:  (1)  Flooding.  (2)  Furrow  distribution. 
(3)  Overhead  sprays.  (4)  Sub-irrigation. 

103.  Flooding. — Flooding  is  practiced  in  several 
ways,  and  is  applied  to  a  much  larger  area  than  any  other 
system.  There  are  two  fundamentally  different  types 
of  flooding:  (1)  One  covers  the  surface  of  the  soil  with 
a  thin  sheet  of  flowing  water,  maintained  until  the 
desired  degree  of  saturation  has  been  reached.  (2)  The 
other  covers  the  surface  with  a  sheet  of  standing  water, 
which  is  allowed  to  remain  until  the  soil  is  sufficiently 
saturated,  when  any  balance  is  drawn  off,  or  may  be 
dissipated  by  percolation  through  the  soil,  as  is  fre- 
quently though  unwisely  done. 

The  former  system  corresponds  closely  with  what  is 
termed  wild  flooding,  where  the  water  is  distributed  by 
a  minute  dendric  system  of  ditches,  and  the  remnant 
gathered  by  a  reversed  dendric  system  of  ditches,  or  by 
a  head  ditch  at  the  foot  of  the  slope.  The  essential 
point  is  to  keep  a  thin  sheet  of  water  moving  over  the 


230        THE  PRINCIPLES    OF   SOIL    MANAGEMENT 

land  until  the  soil  is  saturated.  The  second  system 
agrees  with  check  flooding,  in  which  the  water  is  turned 
on  a  nearly  level  area  to  a  considerable  depth.  The 
check,  or  block,  may  be  a  small  area — a  few  square  rods 
on  a  decided  shape,  or  a  large  area  is  possible  on  very 
level  land.  These  may  be  so  arranged  that  the  water 
flows  successively  from  one  to  the  other,  perhaps  at 
successively  lower  levels.  The  relative  advantages  of  the 
two  types  depend  on  the  character  and  slope  of  the  soil. 
On  gently  sloping  land  of  moderately  porous  character, 
and  not  easily  washed  or  puddled,  so  that  the  water  may 
be  controlled,  wild  flooding  is  the  most  convenient 
method.  Grain  fields  especially  lend  themselves  to  the 
method.  On  the  other  hand,  on  very  level  or  very  steep 
land  the  block  type  must  be  used.  The  water  is  more 
definitely  under  control,  washing  is  largely  prevented 
by  levees,  and  puddling  is  reduced  by  the  almost  entire 
elimination  of  current. 

The  flooding  system  is  best  adapted  to  certain  classes 
of  crops,  as  follows:  (1)  Grain  fields.  (2)  Meadows  and 
hay  fields.  (3)  The  soaking  of  land  preliminary  to  plant- 
ing other  crops,  sometimes  termed  winter  irrigation, 
where  the  water-supply  is  available  only  in  the  winter 
season,  and  is  stored  in  the  soil  until  crop-growing  time. 
The  above  crops  are  adapted  to  occasional  or  intermit- 
tent flooding;  but  some  crops  succeed  best  under  a  con- 
tinual flood  of  water,  as  in:  (4)  Rice  culture  and  (5) 
Cranberry  culture.  A  phase  of  the  flooding  system  is 
the  basin  system  sometimes  used  in  orchard  irrigation. 

The  advantages  of  the  system  are:  (1)  Ease  in  hand- 
ling water.  (2)  Economy  in  irrigation  works.  (3) 


IRRIGATION   BY   FLOODING  AND  FURROWS        231 

Avoids  necessity  of  tearing  up  the  crop  to  form  large 
irrigation  furrows. 

The  objections  to  its  use  are:  (1)  The  large  amount 
of  water  required.  (2)  The  danger  of  over-irrigation, 
with  the  possible  consequent  injury  from  seepage, 
and  the  appearance  of  alkali  salts.  (3)  The  impossi- 
bility of  conserving  water  by  appropriate  cultivation. 
(4)  On  heavy  soils  possible  injury  from  the  crusting 
and  checking  of  the  surface  soil  as  a  result  of  the  lack 
of  tillage.  (5)  Direct  injury  from  flooding  some  crops, 
as  the  potato. 

104.  Furrows. — Furrow  distribution,  by  which,  as 
the  name  implies,  the  water  is  not  applied  to  the  whole 
surface  but  is  distributed  in  furrows.  The  length,  size 
and  arrangement  of  these  depends  directly  on  the 
soil,  chiefly  its  texture.  This  includes  the  subsoil  as  well 
as  the  soil.  In  soils  which  are  porous  or  easily  eroded, 
the  furrows  must  be  shorter  than  where  the  opposite 
conditions  prevail,  in  order  that  the  water  may  reach 
the  further  end  of  the  field  before  over-wetting  the  por- 
tion near  the  head  ditch.  That  is,  in  loose,  porous  soil, 
head  or  feeder  ditches  must  be  nearer  together  than  on 
dense,  impervious  soil. 

The  furrow  system  is  adapted  to  all  intertilled  crops. 
Next  to  the  flooding  system,  it  is  used  on  the  largest 
area,  and  is  adapted  to  all  intensively  cultivated  crops. 
Its  advantages  are  that:  (1)  It  conserves  water.  (2) 
It  is  especially  adapted  to  inter-tilled  crops.  (3)  It 
permits  the  conservation  of  water  by  appropriate  cul- 
tural practices.  (4)  It  avoids  injury  to  crops  sensitive 
to  an  excess  of  water.  Water  should  not  come  in  contact 


232 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


'ING  6X8" 


with  the  trunk  of  trees,  or,  in  general,  with  the  stem  of 
any  plant  not  well  shaded.  A  bright,  warm  sun  in  con- 
junction with  the  excess  of 
water  is  usually  injurious. 

(5)  It  is  the  more  convenient 
method  to  apply  to  the  class 
of  crops  to  which  it  is  adapted. 

(6)  It    more  readily    permits 
the  avoidance  of  the  injuries 
due  to   seepage   by  avoiding 
the   losses    to   which   that    is 
due.    (7)  It  assists  in  the  con- 
trol of  alkali  soils  by  permit- 
ting tillage. 

The  supply  of  soil  mois- 
ture by  capillarity  is  most 
satisfactory  to  the  majority 
of  cultivated  crops,  and  by  promoting  this  the  furrow 
system  generally  gives  better  results  than  flooding. 

The  flooding  system  has  some  disadvantages:  (1) 
It  is  not  so  economical  of  water  as  is  to  be  desired. 
(2)  Much  attention  must  be  given  to  forming  the  furrows, 
to  the  construction  of  head  or  supply  ditches,  to  the 
collection  of  the  overflow  water  at  the  end  of  the  furrows, 
and  in  the  general  supervision  of  the  flow  of  the  water 
over  the  land  to  repair  broken  levees,  etc.  (3)  The  water 
is  not  applied  uniformly.  The  head  of  the  furrow  invari- 
ably becomes  more  wet  than  the  lower  end.  (4)  Erosion 
and  puddling  occur  very  readily  in  cultivated  furrows. 

106.  Overhead  sprays. — Overhead  spray  is  used  only 
on  very  limited  areas,  and  almost  entirely  in  humid 


Fio.  74.  Canvas  dam  with 
opening  to  divide  the  water  in  an 
irrigating  furrow. 


IRRIGATION   BY   SPRAYS  233 

sections.  It  has  been  applied  in  the  growth  of  Sumatra 
wrapper  tobacco  in  Florida,  and  of  truck  crops  near 
New  York,  Boston  and  other  large  cities.  It  is  therefore 
used  as  a  very  limited  supplement  to  the  regular  rainfall. 
It,  is  accomplished  by  the  use  of  a  very  thorough  piping 
system  with  spray  nozzles  at  sufficiently  frequent  inter- 
vals to  cover  the  area.  These  are  connected  with  a  rela- 
tively large  pressure-head  of  water — at  least  five  pounds 
is  necessary. 

The  advantages  of  the  system:  (1)  Economy  in  the 
direct  application  of  water  to  shallow  rooted  crops. 

(2)  Convenience  in  applying  water  at  the  desired  point. 

(3)  Absence  of  injury  from  erosion  or  puddling  the  soil. 

(4)  No  land  wasted  in  irrigation  ditches.    (5)   Natural 
climatic   conditions  developed   by  such  irrigation. 

The  disadvantages  of  the  system  are  great:  (1)  The 
large  initial  cost  of  the  plant.  (2)  The  high  operating 
expenses  ordinarily  necessitated  to  develop  the  pressure 
necessary  to  distribute  water  from  the  nozzles,  and  to 
maintain  the  system.  (3)  The  limited  capacity  of  the 
system.  (4)  The  large  evaporation  from  the  spray  in 
the  atmosphere,  and  from  the  soil  and  surface  of  the 
plants. 

The  spray  system  is  practicable  only  with  special 
crops  under  peculiar  conditions. 

106.  Sub-irrigation. — Sub-irrigation  often  occurs 
naturally.  It  is  the  application  of  water  beneath  the 
surface  of  the  soil.  The  structure  of  the  land  is  such 
that  on  many  low  benches  and  in  river  bottoms  the 
percolation  of  water  through  the  soil  and  fissures  of 
the  rock  brings  it  near  the  surface  at  these  lower  levels, 


234         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

where  it  maintains  a  fairly  constant  supply  of  water  to 
those  crops  which  may  be  growing  on  the  surface. 
The  ground  water  is  so  near  the  surface  in  some  stream 
bottoms,  lake  shores,  etc.,  that  this  condition  prevails. 
Soils  ordinarily  poor  in  their  moisture  relations  become 
highly  satisfactory  in  such  cases.  Sandy  land  is  almost 
ideal  in  its  crop  relations,  so  far  as  moisture  goes,  under 
such  conditions. 

In  a  limited  way  it  has  been  attempted  to  irrigate 
the  soil  from  beneath  the  surface  by  forming  under- 
ground channels  of  porous  pipe,  properly  graded,  into 
which  irrigation  water  may  be  turned,  which  should 
diffuse  through  the  soil  by  percolation  and  capillarity. 

In  some  situations,  as  lawns,  truck  and  fruit  gardens, 
it  may  be  possible  to  install  a  drainage  system  of  tile, 
which  may  also  serve  as  a  means  of  irrigation. 

The  system  has  a  number  of  advantages,  which  in 
ordinary  practice  are  more  than  offset  by  its  disad- 
vantages. Its  advantages  may  be  summarized  as  fol- 
lows: (1)  It  is  very  economical  of  water.  (2)  In  alkali 
soil  it  greatly  reduces  the  surface  accumulation  of 
alkali.  (3)  It  insures  deep  rooting  of  the  crop.  (4)  It 
avoids  waste  land.  (5)  It  avoids  injury  to  the  physical 
condition  of  the  soil.  (6)  Involves  very  little  super- 
vision in  the  application  of  water.  (7)  Possibility  of 
the  use  of  the  system  for  drainage  purposes. 

Its  disadvantages  are:  (1)  The  strong  tendency  of 
roots  to  enter  and  clog  the  pipes.  (2)  The  slow  diffusion 
of  water  by  capillarity  in  dry  soil.  (3)  The  expense 
involved  in  the  installation  of  a  system  of  pipes  ade- 
quate to  irrigate  most  soils. 


SUB-IRRIGATION  235 

Plant  roots  seek  the  most  moist  soil  which  is  short  of 
saturation,  and  therefore  they  are  drawn  toward  and 
tend  to  concentrate  around  and  in  the  lines  of  tile, 
just  as  roots  are  found  to  do  where  drain  tiles  carry  living 
water  through  dry  soil.  This  is  the  greatest  disadvantage 
of  the  system.  Especially  is  this  true  in  orchard  work. 
It  is  more  adapted  to  shallow-rooted  annual  crops,  and 
to  soils  of  strong  rapid  capillary  power,  such  as  fine 
sand  and  coarse  silt  loam  or  loam  soil. 

The  amount  of  water  to  be  added  at  one  time  must 
be  determined  chiefly  by  the  texture  and  structure  of  the 
soil, — or  more  specifically  its  water  capacity, — and  the 
supply  of  water  available.  Under  arid  conditions,  it  is 
generally  advisable  to  apply  as  much  water  as  can  be 
held  within  the  root  zone  by  capillarity  without  loss 
from  percolation.  Frequent  small  applications  should 
be  avoided,  because  of  the  large  proportionate  loss  from 
surface  evaporation.  (See  page  197.)  Also,  there  is  a 
stronger  tendency  to  the  accumulation  of  alkali  salts  at 
the  surface,  because  of  the  larger  evaporation.  On  the 
other  hand,  less  frequent  large  applications  of  water, 
particularly  under  any  but  the  flooding  system,  where 
a  crop  occupies  the  land,  permits  the  creation  and  main- 
tenance of  a  mulch  to  conserve  moisture;  besides  which, 
the  deep  distribution  of  the  water  insures  a  deep  distri- 
bution of  the  roots,  where  they  are  not  only  in  contact 
with  a  larger  moisture  reservoir,  but  also  with  a  larger 
food-supply  than  is  available  to  shallow-rooted  plants. 
It  is  a  fact  of  common  experience  that  in  arid  regions 
crops  generally  root  deeper  than  in  humid  regions. 

A   common   accompaniment   of  irrigation,   certainly 


236 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


in  semi-arid  and  arid  regions,  is  the  excessive  accumu- 
lation of  soluble  salts — "alkali  salts" — in  the  soil. 
They  may  become  so  concentrated  as  to  injure  crops  or 
prevent  their  growth.  (See  page  307.)  In  the  original 
condition  of  such  soils  they  are  usually  distributed  in 
relatively  small  amounts  through  a  deep  section  of  soil. 
But  by  excessive  irrigation,  which  produces  seepage  and 
a  general  rise  in  the  water-table,  aided  by  those  careless 

tillage  methods  which 
permit  free  evaporation 
at  the  surface,  these 
soluble  salts  become  con- 
centrated in  the  root 
zone,  and  at  the  surface 
as  an  alkali  crust.  It 
has  frequently  happened 
that  land  not  originally 
in  a  seriously  "alka- 
line" condition  has  be- 
come so  by  careless 
management.  It  is  obvious  that  to  avoid  this  injury 
there  must  be  (a)  conservative  irrigation,  and  (6)  the 
most  thorough  tillage  methods  which  shall  avoid  surface 
evaporation.  Where  an  excess  of  akali  salts  exists, 
they  are  most  successfully  removed  by  means  of  a  deep 
thorough  drainage  system,  coupled  with  heavy  irrigation 
which  shall  wash  out  of  the  soil  the  excess  of  salts. 

It  is  a  safe  and  wise  rule  to  cultivate  the  soil  as  soon 
after  applying  water  as  its  moisture  condition  will  per- 
mit without  injury,  and  this  should  be  kept  up  at  fre- 
quent intervals  until  an  effective  dust  mulch  has  been 


Fio.  75.  Middle  breaker  plow.  Some- 
times used  in  constructing  irrigation  and 
drainage  ditches. 


PRECAUTIONS   IN   IRRIGATION  237 

created.  It  has  been  noted  (page  204)  that  in  arid 
regions  soil  mulches  are  relatively  more  efficient  and 
more  easily  managed  than  in  humid  regions. 

Soils  of  intermediate  fineness  lend  themselves  most 
readily  to  the  practice  of  irrigation.  Excessively  heavy 
clay  is  generally  to  be  avoided,  because  of  (a)  the  slow 
diffusion  of  water,  by  both  capillarity  and  percolation, 
and  (6)  the  danger  from  puddling  after  an  irrigation, 
unless  cultivation  is  delayed  so  long  that  a  large  amount 
of  water  is  lost.  On  the  other  hand,  very  light  sand  should 
be  avoided  because  of  its  leachy  character,  and  the  great 
loss  of  water  by  percolation  or  surface  evaporation, 
the  former,  if  a  large  amount  of  water  is  added  at  once; 
the  latter,  if  it  is  added  very  frequently. 

But  in  humid  regions  it  is  wise  to  practice  irrigation 
for  crops  easily  injured  by  an  excess  of  water  except  on 
those  light  and  porous  soils  which  have  thorough 
drainage,  because  of  the  possibility  of  a  rainfall  following 
closely  upon  the  application  of  water,  thereby  rendering 
the  soil  over-wet,  to  the  injury  of  the  crop.  On  the  porous 
soil  the  excess  quickly  drains  away.  In  the  Sumatra 
tobacco  region  of  Florida,  for  example,  where  there  is  a 
large  rainfall,  irrigation  has  been  found  successful  only 
upon  the  lighter  sandy  loam  and  sand  soils.  This  crop 
is  particularly  sensitive  to  an  unfavorable  soil  condition. 
Then  too,  the  heavy  soil,  the  clay  loam,  or  clay,  has  a 
large  water  capacity,  which  makes  possible  the  storage 
of  a  large  amount  of  water  against  the  needs  of  the  crop- 
growing  season.  Consequently  it  is  on  these  latter  that 
dry  farming  of  grams  is  most  generally  practiced  in  the 
Western  states. 


238 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


As  the  demand  for  produce  of  high  value  increases, 
the  maintenance  of  the  moisture  supply  of  the  soil  by 
irrigation  may  well  be  extended  on  large  areas  of  soil 
in  so-called  humid  regions,  as  well  as  in  arid  sections. 

The  highest  type  of  soil-management  must  seek  to 
utilize  the  available  water-supply  for  crops  in  the  three 
ways  outlined  above,  that  is,  by  increasing  the  water 
capacity  of  the  soil,  by  eliminating  as  far  as  possible 


Fio. 


An  example  of  poor  drainage  on  level  clay  soil. 


the  losses  by  percolation  and  evaporation  and,  lastly, 
by  supplying  any  deficiency  which  may  still  exist  by 
wise  irrigation. 

107.  Means  of  decreasing  the  water  content  of  the 
soil. — The   removal   of   water   from   the   soil    may   be 
accomplished  in  two  general  ways.   These  depend  upon 
facilitating  the  two  .types  of  loss,  by  percolation  and 
evaporation,    described    on    page    191.     They    are:  (1) 
Drainage.    (2)  Surface  culture,  to  hasten  evaporation. 

108.  Drainage  by  ditches. — Drainage  consists  essen- 


DRAINAGE  239 

tially  in  the  direct  removal  of  the  gravitational  water 
from  the  root  zone  of  the  soil  by  affording  free  passages 
for  its  percolation  and  flow.  In  general,  the  soil  condi- 
tions requiring  drainage  may  be  divided  into  two  groups, 
which  are  fairly  distinct  in  the  problems  which  they 
present.  These  are:  (1)  Those  lands  which  are  satu- 
rated with  water  throughout  the  year.  (2)  Those  lands 
which  are  saturated  with  water  for  only  brief  periods. 
Into  the  first  group  are  placed  all  those  lands  of  an 
acknowledged  swamp  character,  which  not  only  retain 
a  large  part  of  the  water  which  falls  upon  their  own  sur- 
face, but  may  receive  the  water  which  flows  from  other 
lands.  Into  the  second  group  is  put  all  those  wet  lands 
which  are  saturated  for  a  sufficient  period  to  interfere 
with  the  best  condition  of  the  soil,  or  the  proper  develop- 
ment of  the  crop.  It  represents  a  very  mild  or  incipient 
stage  of  the  conditions  included  in  the  first  group. 

In  the  manipulation  of  soil  for  the  staple  upland 
crops,  the  establishment  of  effective  drainage  is  at  the 
foundation  of  all  the  other  practices  which  must  be 
employed.  If  it  does  not  exist,  the  other  farm  practices, 
such  as  tillage,  fertilization  etc.,  can  not  be  applied 
effectively. 

An  excess  of  water  in  the  soil  has  many  and  far-reach- 
ing effects  upon  the  soil  as  a  medium  for  plant  growth, 
especially  if  this  condition  is  intermittent.  The  manage- 
ment of  the  latter  condition  is  even  more  crucial  than 
the  former. 

109.  Effects  of  drainage. — Twelve  of  the  most 
important  effects  of  drainage  are  as  follows:  (1)  Firms 
the  soil.  (2)  Improves  the  granulation.  (3)  Increases 


240         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

the    available    moisture    capacity.     (4)  Improves    the 
aeration  of  the  soil.   (5)  Raises  the  average  temperature. 

(6)  Promotes    the     growth    of    desirable    organisms. 

(7)  Increases  the  available  food  supply.    (8)  Enlarges 
the   root    zone   of   the   soil.     (9)  Reduces    "heaving." 

(10)  Removes     injurious    salts    from     ''alkali     soils." 

(11)  Reduces  erosion.    (12)  Increases  crop  yields,  and 
improves  sanitary  conditions  of  the  region. 

110.  Firms  the  soil. — In  a  saturated  soil  the  particles 
are  held  apart  and  are  partially  floated  by  the  water, 
with  the  result  that  they  afford  a  poor  support  for  plants, 
and  are  largely  unable  to  bear  the  weight  of  travel 
incident   to   cultural    operations.     Heavy   objects   sink 
into  the  surface,  and  become  mired  as  a  result  of  the  easy 
movement    of    the    soil    particles    from    beneath    their 
weight.    This  movement  is  greatly  facilitated  by  the 
lubrication  afforded  by  the  water  between  the  particles. 
It  is  because  of  this  freedom  of  movement  that  a  wet 
soil  may  readily  be  "puddled,"  that  is,  the  small  par- 
ticles moved  into  the  spaces  between  the  large  ones, 
producing  a  more  dense  mass,  a  change  not  possible 
in  dry  or  even  moderately  moist  soils. 

111.  Improves    the    structure. — Drainage    improves 
the  granular  structure  of  fine-textured  soil.    One  of  the 
most  important  factors  in  soil  granulation  is  alternate 
wetting  and  drying.    (See  page  105.)    In  a  wet  soil,  this 
drying  and  drawing  together  does  not  take  place.    On 
the  other  hand,  if  a  granular  soil  be  kept  saturated,  the 
crumb  structure  will  be  broken  down  and  a  bad  physical 
condition  results.    This  is  well  illustrated  by  the  fact 
that  nearly  all  swamp  soils  are  in  a  puddled,  or  otherwise 


DRAINAGE   AND   SOIL   STRUCTURE 


241 


bad  physical  condition,  when  first  drained.  Drainage 
brings  to  bear  upon  the  soil  all  those  natural  agencies 
which  promote  the  granular  arrangement.  In  turn, 
the  granular  structure,  particularly  in  fine-textured 


f  a   20-year-old   tile  drain   in    heavy  clay  soil.     Note  the 
more  open  structure  above  the  drain. 


soil,  affects  the  movement  and  capillary  retention  of 
water,  the  circulation  of  air,  the  growth  of  organisms, 
the  temperature  of  the  soil,  and  other  conditions  depend- 
ent on  these,  in  a  manner  highly  beneficial  to  the  crops 
generally  grown. 

112.  Increases    the    available    water. — Drainage    in- 


242          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

creases  the  available  moisture  capacity  of  fine-textured 
soil.  This  is  accomplished  through  the  better  granulation 
and  larger  porosity  which  results.  The  possibilities  in 
this  direction  are  indicated  by  the  effect  of  structure 
on  the  moisture  capacity  of  the  soil.  (See  page  151.) 
Field  experience  has  many  times  shown  this  result  to 
follow  drainage.  Instead  of  plants  suffering  from  lack 
of  moisture,  as  a  result  of  drainage,  it  is  found  that  they 
are  not  only  free  from  the*  excesses,  but  that  in  dry 
periods  the  soil  is  likely  to  contain  more  moisture  than 
the  same  kind  of  soil  under  poor  drainage.  This  is  especi- 
ally true  of  those  soils,  which  are  wet  only  a  part  of  the 
season.  They  are  subject  to  great  extremes  in  moisture 
content. 

113.  Improves    the     aeration. — Drainage    improves 
the  aeration  of  the  soil  in  two  ways.    (1)  It  removes 
the  gravitational  water  from  the  large  pores,  thereby 
permitting  the  admission  of  air.    (2)  Through  its  effect 
on  granulation  it  permits  the  soil  to  hold  a  larger  volume 
of  air  and  facilitates  its  circulation.    This  also  is  due  to 
two  conditions,  especially  where  the  drainage  is  beneath 
the  surface.   The  larger  pores  resulting  from  granulation 
greatly  aid  the  process.    And  the  underground  passages, 
formed  by  tile  or  other  media,  afford  channels  for  the 
escape  of  soil  air  following  rain  or  reduction  in  baro- 
metric pressure,  and  facilitate  its  readmission  when  the 
opposite  conditions  prevail.    The  net  result  is  a  much 
larger  total  change  between  the  outer  air  and  the  soil 
air.    This  reacts  strongly  upon  the  soil  organisms  and 
upon  the  general  chemical  activity  of  the  soil. 

114.  Raises     the     average     temperature. — Drainage 


DRAINAGE   AND  SOIL   TEMPERATURE  243 

raises  the  average  temperature  of  the  soil.  The  specific 
heat  of  water  is  much  higher  than  that  of  soil,  and  there- 
fore the  larger  proportion  of  water  a  soil  contains  the 
more  heat  is  required  to  increase  its  temperature. 
(See  page  461.)  Further,  in  a  wet  soil  the  surface  evapo- 
ration is  large,  and  since  the  evaporation  requires  several 
hundred  times  as  many  units  of  heat  as  is  necessary 
to  raise  the  same  volume  of  water  from  the  normal 
temperature  to  the  boiling  point,  it  is  clear  that  the 
process  must  consume  a  large  amount  of  heat.  But  the 
heat  supplied  to  any  given  area  of  soil  is  fairly  uniform, 
and  consequently,  if  it  is  used  up  in  evaporating  water, 
it  is  not  effective  to  raise  the  temperature  of  the  soil 
mass.  If  the  soil  contains  water  which  must  be  removed 
by  evaporation,  its  temperature  will  be  kept  correspond- 
ingly low;  or,  what  is  the  same  result,  the  time  required 
to  warm  the  soil  will  be  correspondingly  'extended. 
For  this  reason  a  wet  soil  is  a  "late  soil,"  while  a  well- 
drained  soil  is  much  "earlier"  in  attaining  the  tempera- 
ture necessary  for  the  germination  and  growth  of  plants. 
The  practical  result  of  this  rapid  warming  of  a  well- 
drained  soil  is  to  lengthen  the  growing  season  by  per- 
mitting its  earlier  seeding  in  the  spring,  and  the  later 
growth  of  crops  in  the  fall.  In  some  sections  of  the  world, 
this  margin  in  the  length  of  the  growing  season  deter- 
mines the  growth  of  certain  crops,  and  materially  affects 
all  crops.  All  of  the  activities  of  the  soil,  both  chemical 
and  biological,  are  favorably  affected  by  the  higher 
temperature.  In  the  peat  bogs  of  England,  Parkes  found 
that  at  a  depth  of  seven  inches  the  drained  soil  was  15° 
warmer  than  the  undrained  soil,  and  at  thirty-one 


244         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

inches  it  was  1.7°  warmer.  King  reports  the  frequent 
observation  of  a  difference  of  12°  between  the  tempera- 
ture at  the  surface  of  drained  and  undrained  land. 

115.  Influences    the    growth    of    soil    organisms. — 
Drainage  promotes  the  development  of   the  desirable 
forms  of  organisms,  and  hinders  the  development  of  the 
undesirable  forms.      As  will  be  shown  (page  399),  the 
soil  organisms  may  be  divided  into  two  groups,  one  of 
which  requires  free  oxygen  for  their  growth,  the  other 
does  not.   These  two  groups  are  concerned  with  different 
types  of  chemical   change, — the  one  producing  decay 
the  other  putrefaction.    In  proportion  as  the  air  is  ex- 
cluded by  an  excess  of  water,  normal  decay  is  inhibited 
and  putrefaction  promoted.    The  one  is  beneficial,  the 
other  is  likely  to  be  injurious.    Further,  the  products  of 
the  organisms  accumulate  in  the  excess  of  soil  water  and 
sooner  or  later  may  kill  most  of  the  forms;  as  is  exempli- 
fied in  peat  bogs,  which  owe  their  origin  chiefly  to  this 
fact.    Not  only  is  the  decomposition  of  organic  matter 
retarded,    but    the    chemical    changes    in    the    mineral 
portion  of  the  soil  resulting  from  these  processes  are 
correspondingly  reduced  by  lack  of  drainage.   And  most 
important  of  all  is  the  stimulation  to  the  formation  of 
nitrates  which  results  from  good  drainage.    The  supply 
of  nitrates  is  often  the  controlling  factor  in  plant  growth, 
and  consequently,  in  so  far  as  drain         :ncreases  this 
supply,  it  is  directly  beneficial. 

116.  Increases  the  food-supply. — Drainage  increases 
the  available  food-supply  of  the  soil  in  three  direct  ways: 
(1)  By  holding  in    the  soil  a  larger  proportion  of  avail- 
able moisture  which  favors  a  larger  chemical  activity 


DRAINAGE  AND  THE  ROOT  ZONE        245 

without  removing  the  products  from  the  root  zone.  (2) 
Through  direct  chemical  changes  which  result  from  good 
aeration.  (3)  Through  the  activity  of  organisms  which 
not  only  form  nitrates  but  produce  carbonic  acid  and 
other  materials  which  increase  the  availability  of  the 
mineral  portion  of  the  soil.  The  thoroughness  of  these 
chemical  changes  is  well  illustrated  by  the  uniform  color 
of  a  well-drained  and  well-aerated  soil,  in  contrast  to  the 
usually  mottled  color  of  poorly  aerated  and  wet  soil. 
Drainage  enables  the  plant-grower  to  make  better  use 
of  the  food  stored  in  his  soil. 


FIG.  78.    Cross-section  of  tile-drained  soil,  showing  the  elevation  of  the  water- 
table  between  lines  of  drains. 

117.  Enlarges    the    root    zone. — Drainage    deepens 
and  enlarges  the  root  zone  of  the  soil  by  the  removal 
of  the  gravitational  water  and  by  the  admission  of  air. 
Thereby   the   plant   is   brought   into   intimate   relation 
with  a  much  larger  volume  of  soil  from  which  it  may  draw 
moisture  and  food.    It  is  thus  enabled   o  withstand  more 
protracted  periods  of  dry  weather;  it  enjoys  a  more  uni- 
form climate,  and  has  a  larger  food-supply,  all  of  which 
are  conduciy"        a  rapid  growth  and  a  larger  yield. 

118.  Rf  "heaving." — Drainage  reduces  "  heav- 
ing, "  which  results  from  freezing  of  a  wet  soil.    When 
water  freezes,  it   expands  one-eleventh  of  its   volume. 
In   a  saturated  soil,   this  expansion   can  take   place  in 
only  one  direction — upward — with  the  result  that  the 


246         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

soil  and  consequently  the  crop  is  lifted.  Shallow-rooted 
crops  are  gradually  raised  out  of  the  ground  by  repeated 
freezing  when  wet,  because  the  soil  settled  back  into 


Fio.  79.     Alfalfa  roots  raised  out  of  the  soil   ("heaved")  by  the  repeated 
freezing  of  a  wet  clay. 

place  more  quickly  than  the  root.  Not  only  is  the  plant 
lifted  out  of  the  ground,  but  many  of  the  smaller  roots 
are  broken  off,  all  of  which  greatly  reduces  the  vitality 
of  the  plant.  It  is  most  serious  on  clay  soil,  because  this 


DRAINAGE   AND    "HEAVING"  247 

texture  holds  more  water  and  is  most  likely  to  contain 
an  excess  of  water.  Drainage  reduces  this  type  of  injury 
in  two  ways:  (1)  By  reducing  the  amount  of  water 
present  to  freeze.  (2)  The  larger  volume  of  free  pore 
space,  due  to  the  removal  of  part  of  the  water  and  to  the 
better  granulation,  permits  the  expansion  due  to  freezing 
to  be  taken  up  within  the  mass  of  the  soil,  rather  than 
produce  a  lifting  of  the  surface.  Serious  "heaving" 
is  always  dependent  upon  an  excess  of  soil  water. 

119.  Removes    injurious    salts    from    alkali    soils.— 
Drainage  in  conjunction  with   heavy  irrigation  is  the 
most  effective  means  of  removing  "alkali  salts"  from  arid 
soils.    These  salts  are  dissolved  in  the  irrigation  water 
as  it   passes   through   the  soil,  and   are  then   removed 
in  the  drainage  system  beyond  any  possibility  of  further 
injury.   By  this  practice  it  is  possible  to  reclaim  the  most 
pronounced  areas  of  alkali  soils  to  the  growth  of  the  most 
sensitive  crops. 

120.  Reduces     erosion. — Drainage     reduces     erosion 
due  to  water.    This  type  of  injury  results  from  the  flow 
of  water  over  the  surface.   Drainage  reduces  this  process: 
(1)  By    increasing    the    absorption    of    water.     (2)  By 
affording  channels  in  which  it  may  be  removed  without 
injury,  due  to  a  less  fall,  or  in  conduits  not  subject  to 
erosion,  such  as  tile  drains. 

121.  Increases    crop    yields    and    improves    sanitary 
conditions. — The   direct    practical   result    of   all   of   the 
above  effects   is   larger  and   more   reliable   crop-yields, 
together  with  greater  ease  in  all  cultural  and  harvesting 
operations. 

Coupled  with  the  direct  economic  effect  of  drainage, 


248          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

is  a  large  improvement  in  the  general  sanitary  condi- 
tions of  the  region,  which  was  recognized  long  before 
the  economic  advantages  of  the  practice,  and  has  gener- 
ally been  sufficient  reason  for  public  interest  in  the  prac- 
tice. It  is  only  within  recent  years  that  the  economic 
benefits  of  drainage  have  been  recognized  as  of  sufficient 
public  concern  to  warrant  regulative  legislation. 

122.  Principles  of  drainage. — There  are  two  general 
types  of  drains:  (1)  Open,  or  "surface  drains."  (2)  Cov- 
ered, or  "under  drains."  Each  of  these  types  has  a  partic- 
ular range  of  usefulness  and,  while  they  may  be  substi- 
tuted one  for  the  other  under  some  conditions,  their 
respective  spheres  of  usefulness   are  fairly   distinct. 

123.  Open,    or    surface    drains. — Open    or    surface 
drains  remove  water  from  both  the  surface  and  from 
the  depths  of  the  soil.  Their  efficiency  in  removing  water 
from  the  subsoil   depends  upon  their  depth   and  fall, 
and  upon  the  level  of  water  in  the  channel.    There  are 
certain  conditions  to  which  open  surface  drains  alone 
are  adapted.  These  are:  (1)  Where  the  volume  of  water 
to  be  moved  is  very  large.    (2)  Where  the  water  table 
is  so  near  the  surface,  and  the  fall  so  slight,  that  it  is 
not  possible  to  place  a  drain  below  the  surface.    (3) 
Where  the  drainage  is  designed  to  be  for  only  a  short 
time. 

As  open  ditches  their  efficiency  depends  on  the  sur- 
face flow  of  water  into  their  channel.  They  usually 
tap  the  low  areas  where  the  water  accumulates.  Some- 
times, as  in  river  bottoms,  they  may  be  arranged  regu- 
larly at  intervals,  and  be  of  such  size  as  to  hold  the  water 
which  may  fall  upon  the  surface  during  any  ordinary 


METHODS   OF   DRAINAGE 


249 


rain,  until  such  time,  after  the  subsidence  of  a  general 
overflow  as  it  may  be  removed.  They  may  serve  to 
remove  the  water  accumu'ated  as  the  result  of  an  over- 
flow. In  every  such  case  their  efficiency  depends  upon 
taking  advantage  of  the  natural  inequalities  of  the  sur- 


FIG.  80.     .Surface  ditches  for  drainage  in  a  grain  field.   Such  drains  are  usually 
of  low  efficiency. 

face  of  the  land.  One  phase  of  this  practice  is  to  plow  the 
land  in  narrow  beds,  so  that  the  frequent  "  dead  fur- 
rows" serve  as  surface  drains  and  as  temporary  storage 
for  the  surface  water. 

As  sub-surface  drains,  their  efficiency  depends  upon 
their  depth  being  sufficient  to  permit  percolation  from 


250          THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

the  adjacent  subsoil.  This,  in  turn,  is  determined  by 
the  texture  and  structure  of  the  soil,  and  upon  all  those 
other  factors  which  determine  the  efficiency  of  closed 
drains,  later  to  be  discussed. 

To  be  efficient,  an  open  drain  should  be  properly 
graded,  should  have  a- smooth  bottom  and  sides,  should 
have  sufficiently  tenacious  walls  to  resist  incidental 
erosion,  and  should  have  a  shape  approximately  that 
of  a  semicircle,  which  is  the  form  giving  the  greatest 
carrying  capacity  per  cross-sectional  area.  Since  this 
exact  shape  is  difficult  to  maintain,  it  is  common  in 
practice  to  make  the  depth  and  bottom  width,  respec- 
tively, one-half  the  width  of  the  top,  with  sloping  sides. 
The  form  and  grade  of  the  ditch  must  be  governed  by 
the  character  of  the  soil.  The  steepness  of  the  sides  will 
be  determined  by  the  ability  of  the  soil  to  form  resistant 
walls.  Clay  soil  will  maintain  a  much  steeper  bank  than 
sand.  The  fall  must  not  be  so  great  as  to  produce 
serious  erosion.  A  loam  or  sand  soil  is  much  more  suscep- 
tible to  erosion  than  a  clay.  The  fall  should  be  uniform, 
in  order  that  there  be  no  undue  accumulation  of  sediment 
at  any  point.  Sedimentation  may  be  reduced  by  pre- 
venting the  growth  of  vegetation  in  the  bottom. 

As  deep-soil  drains,  open  surface  ditches  have  a 
number  of  disadvantages,  some  of  which  are:  (1)  They 
are  seldom  of  sufficient  depth.  (2)  As  ordinarily  con- 
structed, they  have  a  small  carrying  capacity,  due  to 
their  uneven  grade  and  rough  bottom  and  sides.  (3) 
They  are  expensive  to  maintain.  (4)  They  waste  much 
land.  (5)  They  greatly  interfere  with  cultural  opera- 
tions. (6)  They  may  be  subject  to  serious  erosion. 


UNDER-DRAINS 


251 


124.  Covered  or  under-drains. — Covered  or  tinder- 
drains  are  any  underground  channels  constructed  for 
the  removal  of  water.  Many  kinds  of  material  have 
been  used  for  this  purpose.  Some  of  the  earlier  materials 
used  were  brush,  stone,  poles,  boards,  and  brick.  In 
recent  years  these  have  been  almost  entirely  supplanted 


FIG.  81.     Construction   of   a   ditch   for    tile   drains. 

by  pipes  made  of  clay  or  cement  because  of  the  greater 
permanency  and  efficiency  of  the  latter. 

The  depth,  frequency  and  size  of  drains  depends  on 
the  character  of  the  soil  and  subsoil,  the  amount  and 
distribution  of  the  rainfall,  the  topography  of  the  sur- 
face, the  crop  to  be  grown,  the  prevalence  of  under- 
ground seepage,  and  the  level  of  (he  ground  water. 
The  system  should  always  be  arranged  with  reference 
to  these  conditions. 


252 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


(a).  Depth.— The  depth  of 
the  drain  must  be  such  that 
the  water  can  find  entrance 
before  it  shall  have  caused 
serious  injury  to  the  crop. 
Since  water  percolates  through 
sand  and  gravel  so  much  more 
readily  than  through  clay, 
drains  may  be  placed  much 
deeper  in  the  former  than  in 
the  latter.  In  coarse-textured 
soil,  drains  attain  their  full 
efficiency  almost  at  once;  but 
in  clay,  owing  to  its  dense 
character  from  long  wetness, 
there  is  a  gradual  increase  in 
efficiency  through  several 
seasons,  as  the  soil  becomes 
better  granulated  and  ac- 
quires other  favorable  struc- 
tural properties.  In  sand, 
water  percolates  rapidly  into 
the  drain,  but  in  clay  this  gen- 
eral movement  is  greatly  re- 
duced and  takes  place  largely 
from  the  sides  and  top  of  the  drain.  In  fact,  a  dense 
clay  soil  holds  its  pores  almost  full  of  capillary  water, 
which  is  not  subject  to  percolation.  Under  such  condi- 
tions, a  large  part  of  the  injury  comes  from  water  stand- 
ing on  the  surface.  Here  the  under-drains  must  be  placed 
very  near  the  surface,  and  function  chiefly  as  surface 


FIG.  82.  Laying  tile  in  the 
bottom  of  ditch  by  use  of  the  tile 
hook.  Shows  arrangement  of  tile 
preparatory  to  rilling  the  ditch. 


CONSTRUCTION   OF    UNDER-DRAINS 


253 


drains.  But,  as  the  excess  of  water  is  removed,  and  the 
soil  structure  is  improved,  they  assume  more  fully  the 
function  of  deep  drains  by  removing  water  from  the 
joints,  or  checks,  which  extend  deeply  into  the  soil. 
Where  deep-rooted  crops  and  trees  are  to  be  grown, 
deeper  drainage  is 
necessary  than  where 
shallow-rooted  crops 
are  grown.  In  gen- 
eral, it  is  not  desir- 
able to  lower  the 
water-table  so  much 
in  sandy  as  in  clay 
soils,  because  of  the 
less  capillary  capacity 
of  the  former.  The 
water-table  should  bp 
lowered  to  from  three 
to  five  feet  below 
the  surface,  but  it  is 
not  always  necessary 
to  place  tile  at  this 
depth,  to  attain  suf- 
ficiently thorough 
drainage.  Where 
there  is  a  distinct 
change  from  sand  to 
clay,  or  vice  versa, 
within  from  two  to 
four  feet  of  the  sur- 

.     .  Flo.  84.     Laying  double-sole  dram- 

,  it  IS  USUally  best  tile  by  hand. 


254 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


to  place  the  drain  on  the  boundary  between  the  two. 
If  the  clay  is  below,  the  water  will  percolate  along  its 
surface  through  the  sand  and  enter  the  tile.  On  the 
other  hand,  if  the  clay  is  underlain  by  sand,  it  is  easier 
for  the  water  to  percolate  downward  into  the  coarse- 
texture  stratum,  and  through  this  into  the  tile,  entering 
from  below. 

(6)  Frequency. — There  are  two  general  systems  of 
arranging   drains:  (1)  The   gridion   or   regular  system. 


Fio.  84.  Two  systems  of  arranging  tile  drains.  Compare  the  amount  of 
double  draining  in  each  system,  due  to  junctions.  Note  the  relative  lengths 
of  tile  required  for  the  same  area  under  each  system . 

(2)  The  natural  or  irregular  system.  In  the  first,  the 
drains  are  arranged  at  definite  regular  intervals  apart,— 
this  interval  depending  chiefly  on  the  texture  of  the  soil. 
This  is  necessary  where  the  surface  is  very  uniform  and 
the  soil  very  homogenous.  It  may  be  applied  to  a  slope 
as  well  as  to  level  land.  In  clay  soil  the  interval  must  be 
less  than  in  coarse-textured  soil.  This  is  because  there 
is  a  drainage  gradient  between  the  drains.  In  fine- 


CONSTRUCTION   OF    UNDER-DRAINS 


255 


S'  3'  S' 


textured  soil  the  water  level  rises  rapidly  «*s's'  s's's* 
away  from  the  drain  and  reaches  the  sur- 
face at  no  great  distance.  On  sand  soil 
this  gradient  is  much  less.  The  aim 
must  be  to  have  the  water  level  reduced 
a  definite  distance  below  the  surface, 
after  a  reasonable  interval  of  time  follow- 
ing rainfall,  and  the  drains  must  be 
sufficiently  frequent  to  accomplish  this. 
In  heavy  clay  soil  this  interval  may  be 
as  small  as  twenty-five  feet,  while  in 
coarse-textured  soil  it  may  be  200  or  300  a 
feet.  Usually,  it  is  best  to  adopt  some 
minimum  interval,  and 
place  the  first  lines  of  tile 
at  two  or  more  times  this 
interval.  If  the  drainage 
does  not  prove  sufficiently  thorough, 
additional  drains  may  be  installed  with- 
out affecting  the  general  system. 

The  natural  or  irregular  system  is 
designed  primarily  to  collect  water  from 
the  surface  where  it  has  accumulated, 
or  beneath  the  surface  where  it  comes 
within  the  range  of  the  plant  roots. 
Large  areas  of  land  are  drained  by  a 
single  line  of  tile  in  the  low  places. 
Where  land  is  kept  wet  by  seepage,  the 

Fio.  86.      Drain       ,       •  ,         ,  ,  ,,  ... 

with       minimum    drains,  should    tap    these    as   near   their 

number     of     large  •  •       ,   , 

tile,    but    having    source  as  is  practicable. 

many     turns     ana  .rl_  •  r       i       •  i 

branches.  Tne   size   of   drains   depends  on   the 


Fio.  85.  A  more 
simple  system  of 
drains,  but  one  re- 
quiring more  large 
tile  than  in  Fig. 88. 


256 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


>o 

Fio.  87.   The  so-called  natural  or  irregular  system  of  arranging  drains  to  remove 
water  from  local  wet  spots.    Shading  indicates  degrees  of  wetness. 

volume  of  water  to  be  handled  and  on  the  fall.  Where 
several  laterals  empty  into  a  main  drain,  the  main  must 
have  a  capacity  equal  to  their  combined  flow;  but  it  is 
not  possible  to  calculate  the  total  or  relative  sizes  with 
the  exactness  which  is  possible  in  a  pressure  system  of 
pipes.  This  is  due  to  the  effect  of  the  soil.  It  acts  as  a 
sponge  to  hold  the  water,  and  gives  it  up  gradually.  The 


258 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


finer  the  soil  the  greater  this  retentive  effect,  and  con- 
sequently the  less  demand  there  is  for  drains  capable  of 
carrying  all  of  the  rainfall  in  a  given  short  time.  Drains 
run  full  for  only  a  very  small  part  of  the  year,  and 
therefore  the  normal  laws  of  hydraulics  are  not  entirely 
applicable  to  them.  In  a  general  way,  doubling  the  fall 
increases  the  carrying  capacity  of  any  given  size  of  tile 
by  one-third.  Where  the  fall  is  less  than  1  per  cent,  it  is 

unwise  to  use  tile  smaller 
than  three  inches  in  di- 
ameter, because  of  their 
strong  tendency  to  clog. 
Water  enters  tile  al- 
most entirely  through 
the  joints  between  the 
sections.  Short  lengths 
are  therefore  better  than 
long  ones.  Through  the 
walls  of  even  soft  brick 
tile  very  little  water  is 
able  to  percolate.  There 
is,  therefore,  no  appreci- 
able advantage  in  using 


FIG.  89.  Hand  tools  used  in  tile- 
drain  construction.  1,  Grade  cord;  2,  pick; 
3,  long-handle,  round-point  shovel;  4 
and  7,  types  of  grading  shovel  for  finish- 
ing the  bottom  of  the  ditch;  5,  spade;  6, 
tile  hook,  used  in  placing  tile  in  ditch  from 
the  bank;  8,  grade  stakes. 


soft  tile,  while  there  are 
their  weakness  and  lia- 


many  disadvantages, — such  as 
bility  to  go  to  pieces  rapidly  under  alternate  wetting 
and  drying,  especially  if  permitted  to  freeze  when 
saturated  with  water. 

Dense,  hard-burned  tile  are  most  safe  to  use  under 
average  soil  conditions. 

"Silting-up"  of  drains  results  where  the  alignment 


DITCHING   MACHINES 


259 


is  bad,  the  joints  too  open,  or  a  section  is  broken.  The 
joints  should  be  fairly  snug,  but  it  is  not  now  considered 
necessary  to  use  collars  in  ordinary  soils.  The  textures 
of  soil  which  give  most  trouble  by  entering  the  joints 
and  stopping  flow  are  very  fine  sand  and  silt.  These 
materials  flow  readily  when  saturated  with  water.  Con- 
sequently, in  laying  tile  in  these  materials,  precaution 
must  be  taken  against  this.  "Silting-up"  is  most  trouble- 


Fio.  90.      Traction  ditching  machine.     A  modern   machine  for  constructing 
tile  ditches.    (See  Fig.  91.) 

some  immediately  after  laying  the  tile,  and  before  the 
soil  structure  has  become  settled  and  readjusted.  When 
this  has  taken  place,  the  tendency  to  silting-up  is  small, 
even  in  fine  sand  and  silt.  In  clay  and  coarse  sand  it  is 
negligible.  This  difficulty  can  be  checked  or  controlled 
by  using  some  filtering  medium  around  the  joints.  Straw, 
leaves,  chaff,  etc.,  are  excellent  and  undergo  slow  decay, 
coincident  with  which  a  resistent  structure  of  soil  is 


UNDER-DRAINS   AND   PLANT   ROOTS 


261 


established.    Fine  gravel  or  coarse  sand  is  a  more  per- 
manent filtering  medium. 

Plant  roots  sometimes  enter  the  joints  of  tile  drains, 
and  develop  so  as  to  stop  the  flow  of  water.  This  occurs 
most  readily  where  the  tile  carries  ''living  water,"  as 
where  a  permanent  spring 
is  drained.  During  dry 
periods  and  in  naturally 
well-drained  soil,  water  per- 
colates from  the  joints  of 
the  tile  into  the  adjacent 
soil,  which  conditions  at- 
tract roots  and  may  lead 
them  into  the  tile  at  the 
joints.  Depth  is  not  a 
decided  protection  against 
this  difficulty  unless  it  be 
excessive. 

There  are  many  points 
about  the  construction  of 
a  tile-drain  system  about 
which  special  precaution 
should  be  taken.  Some  of 
these  are:  (1)  Uniformity 
of  grade.  (2)  Avoid  lead- 
ing a  lateral  into  a  main 
with  a  less  fall  unless  silt 
basins  are  used.  (3)  Pro- 
tection of  outlets  against 

,       ,.  .  Fir,.  92.      Ditch  cut    by  the   ma- 

caving    and    freezing.     (4)     chineshown  in  Fig.91.   soiiaheavy 

ProtCCtion      Of      the      OUtlet       clay.    Depth  4i  feet. 


262 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


against  the  entrance  of  animals.  (5)  Free  flow  of 
water  from  the  outlet.  (6)  Close  joints,  which  may  be 
more  easily  attained  with  round  or  hexagonal  than  with 


FIQ.  93.    A  poorly  constructed  outlet  for  a  line  of  drain  tile. 

U  or  soft  tile.  (7)  Junctions  should  be  made  at  an  acute 
rather  than  at  a  right  angle.  (8)  On  hilly  land  the  drain 
should  run  with  the  slope,  as  far  as  possible.  (9)  In 
general,  the  fall  should  be  as  great  as  the  surface  fea- 


SPECIAL  TYPES  OF  DRAINS          263 

tures  will  permit.  (10)  Avoid  throwing  the  tile  out  of 
alignment  in  filling  the  ditch. 

The  chief  advantages  of  covered  drains,  especially 
when  constructed  of  tile,  are:  (1)  Permanence.  A  well- 
constructed  system  will  last  for  many  decades.  (2) 
Greater  efficiency  where  they  are  suitable.  (3)  No  waste 
of  land.  (4)  No  interference  with  cultural  operations. 
(5)  Require  very  little  care  for  maintenance.  (0)  Less 
cost  over  a  period  of  years. 

126.  Other  types  of  drainage. — Drainage  may  some- 
times be  accomplished  by  means  of  levees.  Where  land 
is  subject  to.  overflow  at  either  frequent  or  infrequent 
intervals,  such  as  river  bottoms  and  tidal  marshes,  their 
drainage  consists  largely  in  excluding  these  inundations. 
Until  this  is  accomplished,  any  other  form  of  drainage 
may  be  useless.  Frequently  direct  drainage  may  advan- 
tageously be  combined  with  some  form  of  levee,  and 
for  tidal  marshes  is  useful  with  the  aid  of  the  fresh 
water  derived  from  rainfall  and  upland  drainage,  in 
removing  its  saltness. 

Wells  or  filter  basins  may  be  used  to  drain  certain 
sinks  or  flat  areas  having  no  other  outlet.  This  is  pos- 
sible only  where  a  very  porous  stratum  occurs  beneath 
the  soil  within  a  reasonable  depth.  Usually  this  is 
practicable  where  a  clay  stratum  is  underlain  by  sand 
or  gravel,  as  occurs  in  many  sections  of  the  country. 
Wells  are  constructed  through  the  clay  to'  the  porous 
stratum,  and  this  may  be  filled  with  stone  or  brish 
as  a  filtering  medium,  and  covered  drains  may  be 
emptied  into  these. 

126.  Surface  culture. — Surface  culture  may  be  em- 


264 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


ployed  to  remove  a  limited  excess  of  water  from  the  soil. 
Those  practices  which  may  be  employed  for  this  purpose 
are  the  opposite  of  those  applied  in  the  conservation  of 
water.  The  most  applicable  ones  are:  (1)  Rolling.  (2) 
Ridged  surface.  (3)  Growth  of  plants. 

Rolling,  or  any  other  practice  which  compacts  the 
soil  and  strengthens  capillary  movement  of  water  to  the 
surface,  places  the  moisture  in  the  most  favorable  position 


Fio.  94.     Water  forced  to  the  surface  by  the  closure  of  the  outlet  of  a  tile  drain. 


REMOVAL   OF    WATER    BY   PLANTS 


265 


for  evaporation.  It  would  be  unwise,  as  a  rule,  to  roll 
the  soil  when  it  is  excessively  wet,  because  of  the  injury 
to  the  structure  of  the  soil  which  would  result.  But  the 


Fie;.  9o.    A  well-constructed  outlet  for  a  line  of  drain  tile. 

land  may  be  rolled  in  anticipation  of  a  wet  period,  which 
condition  of  the  soil  will  facilitate  the  formation  of  that 
compact  surface  which  most  favors  evaporation.  In  the 
spring,  in  regions  of  cold  winters,  bare  or  fallow  land  has 
usually  settled  into  this  condition,  which,  if  permitted 
to  continue,  will  most  rapidly  dry  the  soil. 

Ridging  increases  evaporation  by  exposing  a  larger 


:266 


THE   PRINCIPLES  OF  SOIL   MANAGEMENT 


surface.  In  some  sections  of  the  country  where  the 
wetness  is  most  serious  in  the  spring,  the  crops  are 
planted  on  ridges  which  are  sufficiently  raised  above 
the  general  surface  to  be  drained;  and,  by  the  time  the 
roots  are  ready  to  penetrate  deeply,  the  excess  of  moist- 
ure will  have  been  removed  by  percolation  and  evapo- 
ration. 

Crops  of  any  sort,  including  weeds,  green  manures 
and  cover-crops,  may  serve  to  dry  the  soil  by  evaporating 


Fio.  96.  The  nine  foot  evener  used  in  the  final  filling  of  the  ditch  by  the 
use  of  the  turning  plow  after  laying  drain  tile.  Care  should  be  exercised  in 
placing  the  first  covering  of  earth  over  the  tile  not  to  disturb  their  alignment 
or  break  any  of  the  sections.  This  is  best  accomplished  by  hand,  and  the  earth 
should  be  carefully  pressed  around  the  tile. 

water  from  their  leaves.  It  has  been  seen  (page  134)  that 
the  amount  of  water  so  used  is  large  because  of  the 
functional  activity  of  the  plants  and  the  large  surface 
which  they  expose.  Growing  crops  expand  the  evapo- 
rating surface  of  the  soil  and  are  especially  useful  in 
removing  a  temporary  wetness  in  the  spring. 

The  application  of  any  of  the  above  methods  for  the 
removal  of  water  must  be  guided  by  the  local  conditions 
of  soil,  season,  climate,  crop  and  system  of  farming. 


C.    PLANT    NUTRIENTS   IN   THE   SOIL 

I.     SOLUBILITY    OF    THE  SOIL   THROUGH    NATURAL 
PROCESSES 

Fortunately  for  mankind,  only  an  exceedingly  small 
proportion  of  the  soil  is  at  any  one  time  soluble  in  water 
or  in  the  aqueous  solutions  with  which  it  is  in  contact. 
It  is  this  great  insolubility  that  gives  the  soil  its  perma- 
nence, for,  otherwise,  in  humid  regions,  it  would  be 
rapidly  carried  away  in  the  drainage  water.  The  portion 
soluble  in  the  various  natural  solvents  with  which  it 
comes  in  contact  furnishes  the  mineral-food  materials  for 
plants.  The  great  mass  of  soil  which  is  relatively  in- 
soluble is  constantly  subjected  to  natural  processes 
which  very  slowly  bring  the  constituents  into  solution. 
Those  agents  concerned  in  the  decomposition  of  rock 
also  act  upon  the  soil  to  bring  about  its  further  disin- 
tegration, and  thereby  render  it  more  soluble,  while 
added  to  those  are  the  operations  of  tillage,  which  con- 
tribute to  the  same  end. 

The  surfaces  of  the  particles  alone  come  into  contact 
with  the  decomposing  agents,  and  hence  it  is  these  por- 
tions of  the  particles  that  are  rendered  most  soluble. 
The  factors  that  determine  how  rapidly  solution  shall 
proceed  are:  (1)  The  amount  of  surface  exposed,  which 
we  have  seen  varies  with  the  size  of  the  particles.  (2)  The 
composition -of  the  particles.  (3)  The  strength  of  the 
decomposing  and  solvent  agencies.  Were  it  not  for  this 
process,  there  would  soon  be  no  mineral  food  available 

(267) 


268          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

to  plants,  as  drainage  water  and  the  ash  of  crops  carry 
off  relatively  large  amounts  of  these  substances  each 
year;  but  in  spite  of  this  loss,  the  soil  is  able  to  provide 
at  least  some  plant-food  material  for  each  crop,  when 
called  upon  by  the  plant. 

II.     SOLUBILITY    OF    THE    SOIL    IN    VARIOUS    SOLVENTS 

For  purposes  of  analysis  intended  to  show  the 
amounts  of  mineral  plant-food  materials  in  the  soil  any 
one  of  several  different  solvents  may  be  used.  These 
solvents  differ  in  strength,  and  consequently  the  per- 
centages of  the  various  constituents  obtained  from 
samples  of  the  same  soil  are  different  for  each  solvent. 
A  chemical  analysis  of  a  soil  is  a  determination  of  the 
amounts  of  the  constituents  that  have  been  dissolved 
in  the  solvent  used.  Therefore  it  will  readily  be  seen 
that  the  interpretation  of  a  chemical  analysis  must 
depend  largely  upon  the  nature  of  the  solvent,  and, 
unless  the  solvent  is  equivalent  in  its  action  to  some  pro- 
cess or  processes  in  nature,  the  result  must  be  entirely 
arbitrary.  The  solvents  used  have  generally  been  in- 
tended to  show  some  definite  relation  of  the  soil  to  the 
food  requirements  of  crops.  Upon  the  accuracy  with 
which  this  is  accomplished  depends  the  value  of  the 
chemical  analysis. 

127.  Complete  solution  of  the  soil. — By  the  use  of 
hydrofluoric  and  sulfuric  acids,  the  entire  soil  mass  may 
be  decomposed  and  all  of  its  inorganic  constituents 
determined.  Such  an  analysis  shows  the  total  quantity 
of  the  plant-food  materials  except  nitrogen,  which 


SOLUBILITY   OF  SOIL   CONSTITUENTS  269 

is  never  determined  in  any  of  the  acid  solutions,  but 
by  a  separate  process.  A  deficiency  of  any  particular 
substance  may  be  discovered  in  this  way,  but  nothing 
can  be  learned  as  to  the  ability  of  the  plant  to  obtain 
nutriment  from  the  soil.  A  rock  may  show  as  much 
mineral  plant-food  material  as  a  rich  soil.  Such  an 
analysis  is  used  only  to  ascertain  the  ultimate  limita- 
tions of  a  soil  or  its  possible  deficiency  in  any  essential 
constituent. 

128.  Digestion  with  strong  hydrochloric  acid. — Analy- 
ses made  with  hydrochloric  acid  of  1.115  specific  gravity 
are    those    usually    called    "chemical    soil    analyses." 
They  are  supposed  to  show  the  amount  of  plant  food 
at  the  time  the  analysis  is  made,  which  is  in  a  condi- 
tion to  be  ultimately  used  by  the  plant,  and  the  plant- 
food  materials  not  dissolved  by  treatment  with  hydro- 
chloric acid  are  assumed  to  be  in  a  condition  in  which 
plants  can  not  use  them.    It  may  reasonably  be  ques- 
tioned whether  these  relations  hold  under  field  condi- 
tions.    In   fact,   it  is  quite  certain  that  some  of  them 
do    not    hold.     In    other    words,    while   treatment    With 
hydrochloric  acid  of   a  given  strength  imtrks  a  definite 
point  in  the  solubility  of  the  compounds  in  the  soil,  it 
does  not  bear  a  uniform  relation  to  the  natural  processes 
by    which    these    compounds    become    available    to    the 
plant. 

129.  Interpretation  of  results  of  analysis  of  hydro- 
chloric   acid    solution. — This    method    of    analysis    was 
originally  thought  to  give  some  indication  of  both  the 
permanent  fertility  and  the  immediate  manurial  needs 
of  a  soil;   but   for  both   purposes   the   accuracy   of  the 


270          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

deductions  are  limited  by  a  number  of  conditions  which 
make  it  impossible  to  predict  from  an  analysis  how. 
productive  a  soil  may  be,  or  what  particular  manure 
may  be  profitably  applied.  It  is  very  apparent  that  the 
chemical  composition  of  a  soil  is  only  one  of  the  many 
factors  affecting  its  productiveness.  Unfortunately,  not 
all  of  the  factors  are  understood,  and  consequently 
these  unknown  ones  cannot  be  determined  either  quali- 
tatively or  quantitatively.  If  it  ever  becomes  possible 
to  determine  quantitatively  all  of  the  factors  entering 
into  soil  productiveness  in  the  field  condition,  the  prob- 
lem will  be  solved. 

130.  Permanent    fertility,     and    manurial    needs. — 
Permanent  fertility  can  best  be  judged  by  the  complete 
analysis  of  the  soil,  but,  with  the  exception  of  potash, 
the   possible   deficiency  the   constituents    likely   to   be 
required  in  manures  may  be  judged  from  the  hydro- 
chloric acid  solution  with  a  fair  degree  of  accuracy. 

Conclusions  as  to  the  manurial  needs  of  the  soil  are 
confined  to  ascertaining  whether  any  constituent  is 
present  in  such  small  amount  as  to  furnish  an  inadequate 
supply  for  crop  production.  If,  for  example,  a  certain 
ingredient  is  found  to  be  present  in  very  small  amount, 
it  .may  be  concluded  that  the  addition  of  a  manure  con- 
taining this  substance  would  be  profitable;  but  there  is 
considerable  difference  of  opinion  among  analysts 
as  to  what  this  figure  is  for  each  of  the  ingredients. 
This  minimum  amount  may  vary  with  certain  conditions 
of  soil. 

131.  Relation  of  texture  to  solubility. — The  relative 
amounts  of  sand  and  clay  in  the  soil  and  the  distribution 


INFLUENCE   OF    TEXTURE   ON   SOLUBILITY        271 

of  the  fertilizing  materials  in  these  constituents  will 
affect  the  minimum  amounts  required.  Hilgard  has 
shown  that  the  addition  of  four  or  five  volumes  of  quartz 
sand  to  one  of  a  heavy  but  highly  productive  black  clay 
soil  greatly  increased  the  productiveness,  while  diluting 
the  potash  content  of  the  mixture  to  .12  per  cent  and 
the  phosphoric  acid  to  .03  per  cent.  It  is  evident  that 
in  this  soil  the  plant-food  materials  were  in  a  condition 
to  be  easily  taken  up  by  the  plant  when  the  physical 
condition  of  the  soil  was  suitable. 

If  these  small  amounts  of  food  elements  had  been 
distributed  in  the  sand  particles  as  well  as  in  the  original 
clay,  the  result  would  doubtless  have  been  different. 
Suppose,  for  example,  that  50  per  cent  of  the  potash 
and  phosphoric  acid  had  been  in  the  sand  particles  and 
the  remainder  in  the  clay,  the  former  which  expose 
much  the  less  surface  to  dissolving  liquids  would  be 
proportionately  less  soluble,  and  as  the  minimum  quan- 
tity is  approached,  as  shown  by  the  more  dilute  soil 
yielding  less  than  the  other,  the  effect  would  doubtless 
have  been  to  decrease  the  production.  (See  page  80.) 
In  some  soils,  particularly  those  of  the  arid  region,  the 
larger  particles  may  carry  much  of  the  mineral  nutrients, 
in  which  case  it  is  quite  evident  that  a  higher  percentage 
of  fertility  is  required  than  in  soils  carrying  the  plant  - 
food  material  largely  in  the  small  particles. 

132.  Nature  of  subsoil. — The  nature  and  compo- 
sition of  the  subsoil  is  naturally  a  factor  in  determining 
soil  productiveness,  and  must  be  considered  as  well 
as  the  soil.  An  impervious  subsoil,  or  a  very  loose  sandy 
one,  will  confine  the  productive  zone  largely  to  the  top 


272          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

soil,  and  hence  require  a  greater  proportionate  amount 
of  fertility. 

133.  Calcium  carbonate. — A  determination  of  the 
amount  of  calcium  present  as  a  carbonate  is  important 
as  an  aid  to  the  interpretation  of  an  analysis  of  the  soil. 
Lime  not  so  combined  is  generally  in  the  form  of  a 
silicate,  or  possibly  phosphate.  When  there  is  a  large 
amount  of  calcium  carbonate  in  a  soil,  the  potash,  phos- 
phoric acid  and  nitrogen  are  always  more  readily  soluble, 
and  smaller  quantities  are  sufficient  for  crop  growth  than 
where  the  calcium  is  not  found  in  this  form.  The  effect 
of  the  carbonate  of  lime  upon  the  nitrogen1  com- 
pounds is  to  furnish  a  base  for  the  acids  produced  in  the 
formation  of  nitrates  and  its  presence  promotes  that 
process.  It  probably  replaces  potassium  in  certain 
compounds  where  otherwise  it  would  be  secured  with 
more  difficulty.  It  insures  the  presence  of  some  phos- 
phates of  lime,  in  which  form  phosphorus  is  more 
soluble  than  when  combined  with  iron.  The  form  of  the 
manures  to  be  used  upon  the  soil  will  also  depend  in 
large  measure  upon  the  presence  or  absence  of  calcium 
carbonate.  (See  page  349.)  For  instance,  where  calcium 
carbonate  is  deficient,  steamed  bone  or  Thomas  slag  are 
more  profitable  than  superphosphate,  and  nitrate  of 
soda  than  sulphate  of  ammonium.  Finally,  the  absence 
of  calcium  carbonate  indicates  the  need  of  liming,  and, 
if  the  analyses  show  a  considerable  amount  of  potash 
and  phosphoric  acid,  but  practice  shows  them  to  be 
somewhat  deficient,  it  is  probable  that  liming  will  be 
all  that  is  necessary,  and  that  manures  carrying  these 

'Not  determined  in  the  hydrochloric  acid  extract. 


EXTRACTION    WITH    ORGANIC   ACID  273 

substances  may  be  dispensed  with.  It  must  be  stated, 
however,  that  there  are  cases  for  which  these  deductions 
do  not  hold,  owing  to  the  intervention  of  other  factors. 

134.  Estimation  of  deficiency  of  ingredients. — In   a 
soil  in  which  the  other  conditions  are  normal,  one  would 
suppose  it  possible  to  prescribe,   with  some  degree  of 
accuracy,    the    content    of    certain    constituents    below 
which  a  deficiency  exists.   The  use  of  a  manure  contain- 
ing this   constituent   should   therefore   be   expected   to 
produce    beneficial    results.      However,    opinions    differ 
so  widely,  depending,   apparently,   upon  the  soils  with 
which  the  respective  analysts  have  had  to  deal,  that  it  is 
difficult  to  decide  where  to  set  the  limit.    It  is  evident 
that,  as   the  content  of  any  constituent   becomes  less, 
the  probable  need  for  its  application   becomes  greater, 
and   it   thus    suggests   a   practice    without    assuring  its 
success. 

135.  Conclusions. — An   analysis   of   the   hydrochloric 
acid  extract,  therefore,  cannot   be  taken  as  a  guide  to 
the  fertilizer  needs  of  the  soil,  and  of  itself  should  not 
be  relied  upon;  but  in  connection  with  other  knowledge, 
particularly  that  derived  from  fertilizer  tests,  it  may  be 
useful. 

136.  Extraction    with    dilute    organic    acids.  — Other 
methods  used  for  dissolving  soils   for   analysis   depend 
upon  extraction  with  some  dilute  organic  acid,  as  citric, 
acetic,   oxalic   or  tartaric   acid.     The   assumption   upon 
which  these  methods  are  based  is  that  the  dilute  organic 
acids  correspond  to  the  solvent  agents  in  the  soil,  and 
thus  take  from  it  the  amounts  of  those  materials  that 
the  plant  could  take  up  if  it  came  in  contact  with  all 


274          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

portions  of  the  soil  to  the  depth  represented  by  the 
sample  analysed. 

137.  Advantages  in  showing  manurial   needs. — The 
action  of  each  of  these  dilute  acids  upon  the  same  soil 
does  not  give  equal  amounts  of  the  various  constituents 
in  solution.    Citric  acid  dissolves  especially  lime,  mag- 
nesia and  phosphoric  acid,  and  is  the  most  satisfactory 
solvent    for    purposes  of    analysis.    The    organic    acids 
naturally  dissolve  a  much  smaller  amount  of  material 
from  the  soil  than  does  hydrochloric  acid.    The  former 
acids  permit  the  detection  of  smaller  amounts  of  easily 
soluble  phosphoric  acid  and  potash  than  does  the  latter, 
larger  quantities  of  soil   being  used.     For  example,   a 
chemical  analysis  of  the  hydrochloric  acid  solution  is 
very  likely  not  to  show  any  increase  in  the  phosphorus 
or  potassium  in  a  soil  that  may  have  been  abundantly 
manured  with  these  fertilizers,  and  its  productiveness 
increased  greatly  thereby.    This  is  because  the  amount 
of  plant-food  material  added  is  so  small  in  comparison 
with  the  weight  of  the  area  of  soil  nine  inches  deep  over 
which  it  is  spread  that  the  increase  in  percentage  may 
well  come  within  the  limits  of  analytical  error.    An  acre 
of  soil  nine  inches  deep  weighs  about  2,500,000  pounds. 
If  to  this  be  added  dressings  of  2,500  pounds  phosphoric 
acid  fertilizer  containing  400  pounds  phosphoric  acid, 
it   would  increase  the   percentage  of  that   constituent 
in  the  soil  only  .016  per  cent,  which  difference  could  not 
be  detected  by  the  analysis  of  the  hydrochloric  acid 
solution. 

138.  Usefulness  of  citric  acid. — As  shown  by  Dyer, 
the  use  of  a  1   per  cent  solution  of  citric  acid  is  well 


EXTRACTION  WITH  SOLUTION  OF  CARBON  DIOXIDE  275 

adapted  to  show  the  amount  of  easily  soluble  phosphoric 
acid  and  potash  in  certain  soils,  but  for  other  soils  it  has 
failed  to  give  satisfaction  in  the  hands  of  a  number  of 
analysts.  Shorey,  for  instance,  finds  that  it  fails  utterly 
for  the  highly  ferruginous  soils  of  Hawaii.  It  is,  doubt- 
less, better  adapted  to  soils  rich  in  calcium  and  low  in 
iron  and  aluminum. 

The  reason  urged  by  Dyer  for  the  superiority  of  the 
citric  acid  over  the  hydrochloric  acid  extraction  of  the 
soil  is  that  the  former  gave,  in  his  hands,  several  times 
as  great  a  difference  in  the  amounts  of  soluble  phos- 
phoric acid  in  soils  needing  phosphoric  manures  as  com- 
pared with  those  not  needing  them. 

The  application  of  both  the  hydrochloric  and  citric 
acid  methods  to  a  soil  may,  when  used  to  supplement 
each  other,  add  greatly  to  a  knowledge  of  the  potential 
and  present  productiveness  of  the  soil. 

There  should  be  present  in  a  soil  for  cereals  and  most 
other  crops  at  least  .01  per  cent  phosphoric  acid,  soluble 
in  1  per  cent  citric  acid.  A  soil  containing  less  than  this 
amount  is  deficient  in  phosphoric  acid,  unless  it  exists 
largely  in  the  form  of  ferric  or  aluminum  phosphate, 
which  is  not  readily  soluble  in  citric  acid,  but  is  fairly 
available  to  the  plant.  Sod  land  contains  organic  com- 
pounds of  phosphorus  that  are  easily  soluble  in  the  citric 
acid,  but  less  readily  available  to  the  plant;  hence  such 
soil  should  show  by  analysis  more  than  .01  per  cent 
phosphoric  acid,  to  indicate  sufficiency. 

139.  Extraction  with  an  aqueous  solution  of  carbon 
dioxide. — As  carbon  dioxide  is  a  universal  constituent 
of  the  water  of  the  soil,  and  without  doubt  a  potent 


276 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


factor  in  the  decomposition  of  the  mineral  matter,  it  has 
been  proposed  to  use  a  solution  of  carbon  dioxide  as  a 
solvent  in  soil  analysis.  The  amounts  of  soil  constitu- 
ents taken  up  by  this  solvent  are  much  less  than  by  any 
of  the  others  heretofore  mentioned,  but  all  mineral 
substances  used  by  plants  are  soluble  in  it  to  some  extent. 
The  amount  of  phosphorus  is  so  small  as  to  make  its 


Fio.  97.     The  cut-out  disc  harrow,  adapted  to  hard  or  stony  soil. 


detection  by  the  gravimetric  method  difficult.  Like 
other  methods  employing  very  weak  solvents,  it  is  open 
to  the  objection  that  the  extraction  fails  to  remove  a 
considerable  portion  of  the  dissolved  matter  that  is 
retained  by  absorption,  and,  as  this  will  vary  with  soils 
of  different  texture,  it  makes  impossible  a  fair  com- 
parison of  such  soils  by  this  method. 

140.  Extraction    with    pure    water. — When    soil    is 
digested  with  distilled  water,  all  of  the  mineral  substances 


EXTRACTION    WITH   PURE   WATER  277 

used  by  plants  are  dissolved  from  it,  but  in  very  small 
quantities.  It  has  been  proposed  to  use  this  extract  for 
soil  analysis  on  the  ground  that  it  involves  no  artificial 
solvent,  the  presence  or  amount  of  which  in  the  soil 
is  doubtful,  but  shows  those  substances  which  are  un- 
doubtedly in  a  condition  to  be  used  by  plants.  By 
determining  the  water  content  of  the  soil  and  using 
a  known  quantity  of  water  fd"r  the  extraction,  the  per- 
centage of  the  various  constituents  in  the  soil  water 
or  in  the  dry  soil  may  be  calculated. 

The  substances  dissolved  from  the  soil  by  extraction 
with  distilled  water  are  probably  only  those  contained 
in  the  soil-water  solution,  including  a  part  of  the  solutes 
held  by  absorption.  The  aqueous  extract  does  not  con- 
tain all  of  the  nutritive  salts  in  solution  in  the  soil  water, 
and  hence  is  not  a  measure  of  the  fertility  held  in  that 
form.  An  undetermined  amount  of  nutrients  is  retained 
in  the  water  in  the  very  small  spaces  and  on  the  surface 
of  the  soil  particles.  It  is,  however,  a  fair  comparative 
measure  of  the  content  of  available  nutrients. 

141.  Influence  of  absorption. — The  quantity  of  ex- 
tracted material  depends  upon  the  absorptive  properties 
of  the  soil,  and  upon  the  amount  of  water  used  in  the 
extraction,  or  upon  the  number  of  extractions.  Analyses 
of  the  aqueous  extract  of  a  clay  and  of  a  sandy  soil  on 
the  Cornell  University  Farm  serve  to  illustrate  the 
greater  retentive  power  of  the  former  for  nitrates. 
Sodium  nitrate  was  applied  to  a  clay  soil,  and  to  a  sandy 
loam  soil  at  the  rate  of  040  pounds  per  acre.  Analyses 
of  aqueous  extract,  some  ninety  days  later,  showed 
the  following: 


Kind  of  soil 

Fertilizer 

Nitrates  in  soil. 
Parts  per  million 

Clay  . 

Sodium  nitrate 

7.8 

Clay  

No  fertilizer 

1.8 

Sandy  loam  

Sodium  nitrate 

1500 

Sandy  loam  

No  fertilizer 

29.7 

There  was  apparently  a  much  greater  retention  of 
nitrates  by  the  clay  soil,  as  shown  by  a  comparison  of 
the  fertilized  and  unfertilized  plats  on  both  soils. 

Schulze  extracted  a  rich  soil  by  slowly  leaching  1,000 
grams  with  pure  water,  so  that  one  liter  passed  through 
in  twenty-four  hours.  The  extract  for  each  twenty-four 
hours  was  analyzed  every  day  for  a  period  of  six  days. 
The  total  amounts  dissolved  during  each  period  were 
as  follows: 

TABLE  XLII 


Successive  extractions 

Total  matter 
dissolved 

Volatile 

Inorganic 

First  

.535 

.340 

.195 

Second  

.120 

.057 

.063 

Third   

.261 

.101 

.160 

Fourth  

.203 

.083 

.120 

Fifth  

.260 

.082 

.178 

Sixth  

.200 

.077 

.123 

It  will  be  noticed  that  the  dissolved  matter,  both 
organic  and  inorganic,  fell  off  markedly  after  the  first 
extraction,  which  was  larger  on  account  of  the  matter 
in  solution  in  the  soil  water.  Later  extractions  were 
doubtless  supplied  largely  from  the  substances  held 
by  absorption  and  which. gradually  diffuse  into  the  water 


INFLUENCE   OF   ABSORPTION   ON   SOLUBILITY     279 

extract,  as  the  tendency  to  maintain  equilibrium  of 
the  solution  overcomes  the  absorptive  action.  With  the 
removal  of  the  adsorbed  substances,  the  equilibrium 
between  the- soil  particles  and  the  surrounding  solution 
is  disturbed,  solvent  action  is  increased,  and  more 
material  gradually  passes  from  the  soil  into  the  solution. 
In  this  way  the  uniform  and  continuous  body  of  extrac- 
tives is  maintained. 

142.  Other  factors. — For  purposes  of  soil  analysis, 
the  quantity  of  water  used  for  extraction  must  be  placed 
at  some  arbitrary  figure,  and  the  method  is  open  to  the 
objection  that  it  does  not  represent  accurately  the  soil 
water  solution.  Analyses  of  soils  of  different  types  are 
not  comparable,  and  the  water  extract  cannot  be  con- 
sidered to  measure  the  concentration  or  even  the  com- 
position of  the  solution  existing  between  the  root  hair 
and  the  soil  particles.  However,  for  studying  some  of 
the  changes  that  go  on  in  the  soil,  and  which  are  detect- 
able in  the  soil-water  solution,  the  method  may  be  used 
to  advantage. 

III.  MINERAL  SUBSTANCES  ABSORBED  BY  PLANTS 

The  plant,  in  its  process  of  growth,  withdraws  from 
the  soil  certain  mineral  matters  that  are  presented  to 
its  roots  in  a  dissolved  condition.  As  the  salts  in  solution 
are  quite  numerous,  and  as  the  osmotic  process  by  which 
the  absorption  is  accomplished  does  not  admit  of  the 
entire  exclusion  of  any  substance  capable  of  diosmosis, 
there  are  to  be  found  in  the  plant  most  of  the  mineral 
constituents  of  the  soil.  Some  of  these  are  concerned  in 


280         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

the  vital  processes  of  the  plant  and  are  essential  to  its 
growth.  Others  seem  to  have  no  specific  function,  but 
are  generally  present. 

143.  Substances  found  in  ash  of  plants. — The  sub- 
stances commonly  met  with  in  the  ash  of  plants  are 
potassium,  sodium,  calcium,  magnesium,  iron,  aluminum, 
phosphorus,  sulfur,  silicon,  and  chlorine.  In  addition 
to  these,  nitrogen  is  absorbed  from  the  soil  in  the  form 
of  soluble  salts. 

The  substances  known  to  be  absolutely  essential  to 
the  mature  growth  of  plants  are  potassium,  calcium, 
magnesium,  iron,  phosphorus,  sulfur  and  nitrogen, 
while  the  others  are  probably  beneficial  to  the  plant  in 
some  way  riot  yet  discovered. 

Of  the  substances  acting  as  plant  nutrients,  each  must 
be  present  in  an  amount  sufficient  to  make  possible  the 
maximum  growth  consistent  with  other  conditions,  or 
the  yield  of  the  crop  will  be  curtailed  by  its  deficiency. 
To  some  extent  certain  essential  substances  may  be 
substituted  by  others,  as,  for  instance,  potassium  by 
sodium;  but  such  substitution  is  probably  possible  only 
in  some  physiological  role  other  than  that  of  an  ele- 
mental constituent  of  an  organic  compound.  The  sub- 
stances that  are  likely  to  be  so  deficient  in  an  available 
form  in  any  soil  as  to  curtail  the  yield  of  crops  are  potas- 
sium, phosphorus  and  nitrogen,  while  the  addition  of 
certain  forms  of  calcium  is  likely  to  be  beneficial  on 
account  of  its  relation  to  other  constituents  and  proper- 
ties of  the  soil.  It  is  for  the  purpose  of  supplying  these 
substances,  and  to  some  extent  to  improve  the  mechani- 
cal condition  of  the  soil,  that  mineral  manures  are  used. 


PLANT -FOOD   REMOVED    BY    CROPS 


281 


144.  Amounts  of  plant-food  material  removed  by 
crops. — The  utilization  of  mineral  substances  by  crops 
is  a  source  of  loss  of  fertility  to  agricultural  soils.  In 
a  state  of  nature,  the  loss  in  this  way  is  comparatively 
small,  as  the  native  vegetation  falls  upon  the  ground, 
and  in  the  process  of  decomposition  the  ash  is  almost 
entirely  returned  to  the  soil.  Under  natural  conditions, 
soil  usually  increases  in  fertility;  for,  while  there  is  some 
loss  through  drainage  and  other  sources,  this  is  more 
than  counterbalanced  by  the  action  of  the  natural 
agencies  of  disintegration  and  decomposition,  and  the 
fixation  of  atmospheric  nitrogen  affords  a  constant, 
although  small  supply,  of  that  important  soil  ingredient. 


Fio.  98.  A  collection  of  hand-tillage  implements.  From  left  to  right: 
1.  Field  and  garden  hoe.  2.  Mattock.  3.  Weeding -hoe.  4.  Stone-hoo';s. 
5.  tinper-weeder.  6.  Grub-hoe.  7.  Scuffle-hoe.  8.  Garden  rake.  9.  Spading- 
fork.  10.  Garden  trowel. 


282 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


When  land  is  put  under  cultivation,  a  very  different 
condition  is  presented.  Crops  are  removed  from  the 
land,  and  only  partially  returned  to  it  in  manure  or 
straw.  This  withdraws  annually  a  certain  small  propor- 
tion of  the  total  quantity  of  mineral  substances,  but, 
what  is  of  more  immediate  importance,  it  withdraws 
all  of  this  in  a  readily  available  form. 

The  following  table,  computed  by  Warington,  shows 
the  amounts  of  nitrogen,  potassium,  phosphorus  and 
lime  removed  from  an  acre  of  soil  by  some  of  the  common 
crops.  The  entire  harvested  crop  is  included. 

TABLE  XLIII 


Crop 

Yield 

Total 
ash 

Nitro- 
gen 

Potash 

Lime 

Phos- 
phoric 
acid 

Wheat   . 

30  bus. 

Pounds 
172 

Pounds 
48 

Pounds 

28.8 

Pounds 
9.2 

Pounds 
21.1 

Barley 

40  bus. 

157 

48 

35.7 

9.2 

20.7 

Oats  

45  bus. 

191 

55 

46.1 

11.6 

19.4 

Maize  

30  bus. 

121 

43 

36.3 

18.0 

Meadow  hay.  . 
Red  clover  .  .  . 
Potatoes    .... 
Turnips  

1£  tons 
2  tons 
6  tons 
17  tons 

203 
258 
127 
364 

49 
102 
47 
192 

50.9 
83.4 
76.5 
148.8 

32.1 
90.1 
3.4 
74.0 

12.3 
24.9 
21.5 
33.1 

145.  Amounts  of  plant-food  materials  contained 
in  soils. — Comparing  the  figures  given  above  with  those 
showing  the  total  amounts  ,of  the  fertilizing  constituents 
in  certain  soils,  it  is  evident  that  there  is  a  supply  in 
most  arable  soils  that  will  afford  nutriment  for  average 
crops  for  a  very  long  period  of  time.  The  following 
table  shows  the  amount  of  nutrients  contained  in  the 
chief  divisions  of  soil  as  given  on  page  30. 


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POSSIBILITY   OF   EXHAUSTING   SOILS  285 

146.  Possible  exhaustion  of  mineral  nutrients. — 
On  the  other  hand,  when  we  consider  that  the  soil  must 
be  depended  upon  to  furnish  food  for  humanity  and 
domestic  animals  as  long  as  they  shall  continue  to 
inhabit  the  earth,  at  least  so  far  as  we  now  know,  the 
very  apparent  possibility  of  exhausting,  even  in  a  period 
of  several  hundred  years,  the  supply  of  plant  nutrients 
becomes  a  matter  of  grave  concern.  The  visible  sources 
of  supply,  to  replace  or  supplement  those  in  the  soils  now 
cultivated  are,  for  the  mineral  substances,  the  subsoil 
and  the  natural  deposits  of  phosphates,  potash  salts, 
and  limestone,  and  for  nitrogen  deposits  of  nitrates, 
the  by-product  of  coal  distillation  and  the  nitrogen 
of  the  atmosphere.  The  last  of  these  is  inexhaustible, 
and  the  exhaustion  of  the  nitrogen  supply,  which  a 
few  years  ago  was  thought  to  be  a  matter  of  less  than 
half  a  century,  has  now  ceased  to  cause  any  apprehen- 
sion. The  conservation  or  extension  of  the  supply  of 
mineral  nutrients  is  now  of  supreme  importance.  The 
utilization  of  city  refuse  and  the  discovery  of  new 
mineral  deposits  are  developments  well  within  the  range 
of  possibility,  but  neither  of  these  promises  to  afford 
more  than  partial  relief.  The  utilization  of  the  subsoil 
through  the  gradual  removal  by  natural  agencies  of  the 
top  soil  will,  without  doubt,  tend  to  constantly  renew  the 
supply.  The  removal  of  top  soil  by  wind  and  erosion  is, 
even  on  level  land,  a  very  considerable  factor.  The  large 
amount  of  sediment  carried  in  streams  immediately  after  a 
rain,  especially  in  summer,  gives  some  idea  of  the  extent 
of  this  shifting.  This  affects  chiefly  the  surface  soil  and 
thereby  brings  the  subsoil  into  the  range  of  root  action. 


286         THE   PRINCIPLES   OF  SOIL  MANAGEMENT 

IV.     ACQUISITION    OF    NUTRITIVE    SALTS    BY    AGRICUL- 
TURAL   PLANTS 

All  of  the  salts  taken  up  by  the  roots  of  agricultural 
plants  are  in  solution  when  absorbed.  The  movement 
into  the  root  thus  depends  upon  the  presence  of  moisture, 
which  is  the  medium  of  transfer.  The  root  hairs  are  the 
great  absorbing  portions  of  the  plant,  and  through  the 
cells  of  their  delicate  tissues  the  solutions  of  the  various 
salts  pass  by  osmotic  action.  (See  Fig.  53.)  The  nature 
and  quantity  of  material  absorbed  is  determined  by  the 
law  of  osmosis.  From  the  cells  of  the  root-hairs  the  dis- 
solved salts  are  transferred  to  other  portions  of  the  plant, 
where  they  undergo  the  metabolic  processes  that  deter- 
mine which  constituents  shall  be  retained  in  the  tissues 
of  the  plant.  The  unused  ions  which  remain  in  the  plant 
juices  prevent  by  their  presence  the  further  absorption 
of  those  particular  substances  from  the  soil  water.  It  thus 
happens  that  the  composition  of  the  ash  of  a  plant  may 
be  very  different  from  that  of  the  substances  presented 
to  it  in  solution.  For  instance,  aluminum,  although 
always  present  in  the  soil,  in  a  very  slightly  soluble  form, 
is  either  absent  or  present  in  mere  traces  in  the  ash  of 
most  plants.  On  the  other  hand,  iodine,  although  prese-nt 
in  sea-water  only  in  the  most  minute  amounts,  is  present 
in  large  quantities  in  the  ash  of  certain  marine  algae. 

147.  Selective  absorption. — A  plant  will,  in  general, 
take  up  more  of  a  nutritive  substance  when  presented 
in  large  amount,  as  compared  with  the  other  soluble 
substances  in  the  nutrient  solution,  than  if  presented 
in  small  amount.  Thus,  the  percentage  of  nitrogen  in 


ACQUISITION   OF  NUTRIENTS   BY  PLANTS         287 

maize,  oats  and  wheat  may  be  increased  by  increasing 
the  ratio  of  nitrogen  to  other  nutritive  substances  in 
the  nutrient  media.  This  is  also  true  of  potassium  and 
phosphorus,  respectively.  This  fact  is  accounted  for 
by  the  maintenance  of  the  osmotic  equilibrium  at  a 
higher  level  for  a  particular  ion  which  is  relatively 


Fio.  99.    Showing  the  intimate  relation  of  root-hairs  and  soil  particles. 

abundant  in  the  nutrient  solution,  thus  preventing  the 
return  of  the  excess  from  the  plant. 

148.  Relation  between  root-hairs  and  soil-particles.— 
In  a  rich,  moist  soil  the  number  of  root-hairs  is  very 
great,  while  in  a  poor  or  a  dry  soil  there  are  compara- 
tively few.  The  connection  between  the  root-hairs  and 
the  soil-particles  is  extremely  intimate.  When  in  con- 
tact with  the  particle  of  soil,  the  root-hair  frequently 
almost  incloses  it,  and  by  means  of  its  mucilaginous 


288          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

wall  forms  a  contact  so  close  as  to  practically  make  the 
solution  between  the  particle  and  the  cell-wall  distinct 
from  that  between  the  soil-particles. 

There  has  been  considerable  difference  of  opinion 
as  to  how  the  plant  can  obtain  its  mineral  nutrients 
from  a  substance  so  difficultly  soluble  as  the  soil.  It  has, 
of  course,  been  recognized  that  the  soil-water  is  aided 
in  its  solvent  action  by  a  variety  of  substances  that  may 
be  normally  present  in  solution,  beginning  with  the 
gases  taken  up  by  the  rain  in  its  descent  through  the 
atmosphere,  and  further  aided  by  the  carbon  dioxide 
and  organic  and  mineral  substance  obtained  from  the 
soil.  It  has  been  held  that  the  plant-roots  aid  solution 
of  mineral  matter  by  excretion  of  acids,  which  act  effec- 
tively as  solvents.  The  well-known  root-tracings  on 
limestone  and  marble  have  been  taken  as  proof  of  the 
excretion  of  such  acids.  Sachs  and,  later,  other  investi- 
gators grew  plants  of  various  kinds  in  soil  and  other 
media  in  which  was  placed  a  slab  of  polished  marble 
or  dolomite  or  calcium  phosphate,  covered  with  a  layer 
of  washed  sand.  After  the  plants  had  made  sufficient 
growth,  the  slabs  were  removed  and  on  the  surfaces 
were  found  corroded  tracing,  corresponding  to  the  lines 
of  contact  between  the  rootlets  and  the  minerals. 

In  order  to  test  this  theory,  Czapek  repeated  the 
experiments,  using  plates  of  gypsum  mixed  with  the 
ground  mineral  that  he  wished  to  test,  and  this  mixture 
he  spread  over  a  glass  plate.  Using  these  plates  in  the 
same  manner  as  previously  described,  Czapek  found 
that,  while  plates  of  calcium  carbonate  and  of  calcium 
phosphate  were  corroded  by  the  roots,  plates  of  alumi- 


ROOT   EXCRETIONS   AND  SOLUBILITY  289 

num  phosphate  were  not.  He  concludes  that  if  the  trac- 
ings are  due  to  acids  excreted  by  the  plant-roots,  the 
acids  so  excreted  must  be  those  that  have  no  solvent 
action  on  aluminum  phosphate.  This  would  limit  the 
excreted  acids  to  carbonic,  acetic,  propionic  and  butyric. 
Czapek  also  replies  to  the  argument  that  the  acids  pro- 
ducing the  tracings  must  be  non-volatile  ones,  because 
of  the  definite  lines  made  in  the  mineral,  by  stating 
that  the  excretion  of  carbon  dioxide  alone  would  be 
sufficient  to  account  for  the  observations,  as  it  dissolves 
in  water  to  form  carbonic  acid,  and  that  carbonic  acid 
is  always  present  in  the  cell-walls  of  the  root  epidermis, 
from  which  it  does  not  readily  exude. 

Czapek  has  also  shown  that  liquids  having  an  acid 
reaction  exude  from  root-hairs,  and  he  attributes  the 
reaction  to  the  presence  of  acid  salts  of  mineral  acids, 
having  found  potassium,  phosphorus,  magnesium,  cal- 
cium and  chlorine  in  this  exudate.  He  has  not  proven, 
however,  that  the  exudations  were  not  from  dead  root- 
hairs,  or  from  the  dead  cells  of  the  root-cap.  In  either 
case  they  would  have  some  solvent  action,  but  whether 
sufficient  to  make  them  of  importance  it  is  impossible 
to  say. 

Kunze,  who  followed  up  this  work,  discredits  the 
theory  of  excretion  of  acid  salts  of  mineral  acids,  and 
attributes  the  corrosive  action  of  roots  to  organic  acids. 
In  his  experiments  with  200  species,  he  found  that  many 
plants  do  not  excrete  enough  acid  to  be  detected  by 
litmus.  He  attributes  to  fungi  much  the  greater  activity 
in  this  respect,  and  considers  them  more  important  in 
disintegrating  the  soil  than  are  the  higher  plants. 


290        THE  PRINCIPLES    OF    SOIL    MANAGEMENT 

The  present  status  of  experimental  evidence  on  excre- 
tion of  acids  other  than  carbonic  by  the  roots  of  plants 
does  not  admit  of  any  very  satisfactory  conclusion  as  to 
their  relative  importance  in  the  acquisition  of  plant-food 
materials.  There  can  be  no  doubt,  however,  that  carbon 
dioxide,  resulting  from  root  exudation,  and  from  decom- 
position of  organic  matter  in  the  soil,  plays  a  very  promi- 
nent part  in  this  operation.  The  very  large  quantity 
of  carbon  dioxide  in  the  soil,  amounting  in  some  cases 
to  from  5  to  nearly  10  per  cent  of  the  soil  air,  or  several 
hundred  times  that  of  the  atmospheric  air,  must  aid 
greatly  in  dissolving  the  soil-particles. 

Whatever  may  be  the  concentration  of  the  soil-water, 
it  seems  probable  that  the  liquid  to  be  found  where  the 
root-hair  comes  in  contact  with  the  soil-particle,  and 
which  is  separated,  in  part  at  least,  from  the  remainder 
of  the  soil-water,  must  have  a  density  much  greater  than 
that  found  elsewhere  in  the  soil.  The  comparatively 
rich  juices  of  the  plant  separated  from  the  soil  water 
only  by  the  delicate  cell-walls  of  the  root-hair  insures 
a  copious  transfer  of  the  constituents  of  these  juices 
into  the  intervening  water,  thus  bringing  into  contact 
with  the  soil  mineral  salts,  of  which  some  are  doubtless 
acid  salts  and  also  mineral  salts  of  organic  acids,  and, 
possibly,  some  free  organic  acids.  That  portion  of  the 
soil-water  immediately  in  contact  with  the  soil  grain  is 
a  much  stronger  solution  than  the  water  further  from 
the  soil  surfaces  on  account  of  the  absorptive  action  of 
the  particles.  These  solutions,  coming  in  contact  with 
the  surface  of  the  soil-particles  already  subjected  to  the 
bacterial  and  other  disintegrating  agents  of  the  soil, 


ABSORPTIVE   POWER   OF   DIFFERENT   CROPS       291 

may  readily  be  conceived  to  start  an  active  transfer 
of  mineral  substances  into  the  plant. 

Plants  grown  in  solutions  of  nutritive  salts  have 
few  or  no  root-hairs,  but  absorb  through  the  epidermal 
tissue  of  the  roots.  If  the  plant  depended  upon  the  pre- 
pared solution  in  the  soil-water,  a  similar  structure  would 
doubtless  suffice.  The  special  modification  by  which 
the  root-hairs  come  in  intimate  contact  with  the  soil- 
particle,  and  almost  surrounds  it,  indicates  a  direct 
relation  between  the  soil-particles  and  the  plant,  and  not 
merely  between  the  soil  solution  and  the  plant. 

New  root-hairs  are  constantly  being  formed,  and  the 
old  ones  become  inactive  and  disappear.  The  contact 
of  a  root-hair  with  a  soil-particle  is  not  long-continued. 
Whether  the  period  of  contact  is  determined  by  the 
ability  of  the  root  to  absorb  nutriment  from  the  particle 
is  not  known.  Certain  it  is  that  only  a  small  portion  of 
the  particle  is  removed.  It  may  be  true  that  only  the 
immediate  surface  which  had  been  previously  acted 
upon  by  the  disintegrating  agents  of  the  soil,  and  thus 
rendered  more  easily  soluble,  is  affected  by  the  absor- 
bent action  of  the  root-hairs. 

149.  Absorptive  power  of  different  crops. — As  has 
already  been  pointed  out  (page  281),  crops  of  different 
kinds  vary  greatly  in  their  ability  to  draw  nourishment 
from  the  soil.  The  difference  between  the  nitrogen, 
phosphorus  and  potassium  taken  up  by  a  corn  crop 
of  average  size  and  a  wheat  crop  of  average  size  is  very 
striking.  Corn  has  the  longer  growing  period,  but  as  be- 
tween oats  and  wheat,  where  the  growing  period  is  nearly 
identical,  a  similar  relation  exists. 


292         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

The  difference  in  absorbing  power  may  be  due  to 
either  one  or  both  of  two  causes:  (1)  A  larger  absorbing 
system.  (2)  A  more  active  absorbing  system.  The 
former  is  determined  by  the  extent  of  the  root-hair 
surfaces;  the  latter  by  the  intensity  of  the  osmotic  action. 

160.  Extent     of     absorbing     system. — Plants     with 
large  root  systems  may,  therefore,  be  expected  to  absorb 
the  larger  amounts  of  nutrients  from  the  soil,  and  such 
is  usually  the  case,  although  the  extent  of  the  root-system 
is  not  necessarily  proportional  to  the  total  area  of  the 
absorbing  surfaces  of  the  root-hairs. 

161.  Osmotic   activity. — The  osmotic   activity   of   a 
plant  under  any  given  condition  of  soil   and  climate 
depends  upon:  (1)  The  rapidity  and  completeness  with 
which  the  plant  elaborates  the  substances  taken  from 
the  soil  into  plant  substance  or  otherwise  removes  them 
from  solution.    (2)  The  extent  to  which  the  exudations 
from  the  root-hairs  act  upon  the  soil  particles  to  in- 
crease the  density  of  the  solution  between  the  root-hair 
and  the  soil-particle. 

The  first  of  these  is  a  function  of  the  vital  energy 
of  the  plant  and  its  ability  to  utilize  sunshine  and  carbon 
dioxide  to  produce  organic  matter.  It  may  be  com- 
pared to  the  property  which  enables  one  animal  to  do 
more  work  than  another  animal  of  the  same  weight  on 
a  similar  ration. 

The  removal  from  the  ascending  water  current  in 
the  plant  of  substances  derived  from  the  soil  is  accom- 
plished in  the  leaves.  By  the  dissociation  of  these,  ions 
are  constantly  furnished  for  metabolism  into  materials 
that  may  be  built  into  the  tissues  of  the  plant.  The 


FACTORS   AFFECTING   ABSORPTIVE   POWER        293 

remaining  ions  are  kept  in  the  solution.  There  is  a  con- 
stant tendency  to  bring  the  composition  and  density 
of  the  solution  into  equilibrium,  by  diffusion  and  dios- 
mosis,  with  the  solution  between  the  soil-particle  and  the 
root-hair.  The  rapidity  with  which  the  metabolic  pro- 
cess removes  a  substance  from  the  solution  in  the  plant, 
therefore,  determines  the  rate  at  which  it  is  removed 
from  a  solution  of  given  composition  and  density  in  the 
soil.  Plants  making  a  rapid  growth  remove  more  nutri- 
ents in  a  given  time  than  those  making  a  slower  growth, 
when  the  nutrient  solution  is  of  a  given  composition 
and  density.  A  maize  plant,  for  instance,  removes  more 
nutriment  from  a  given  solution  in  one  day  during  its 
stage  of  most  rapid  growth  than  does  a  wheat  plant 
during  a  corresponding  stage. 

Another  factor  which  affects  the  rate  of  absorption 
of  salts  from  the  soil  is  the  solvent  influence 'of  exudates 
from  the  root-hairs.  This  subject  has  already  been 
treated  (page  287),  and  it  only  remains  to  say  that  this 
action  apparently  varies  with  different  kinds  of  plants, 
and  probably  accounts  in  no  small  measure  for  the  dif- 
ference in  the  ability  of  different  plants  to  withdraw 
salts  from  the  soil. 

These  several  factors,  which,  when  combined,  deter- 
mine the  so-called  "feeding-power"  of  the  plant,  are 
recognized  by  the  popular  terms  "weak-feeder"  and 
"strong-feeder," — applied,  on  the  one  hand,  to  such 
crops  as  wheat  or  onions,  which  require  very  careful 
soil  preparation  and  manuring,  and,  on  the  other  hand, 
to  maize,  oats  or  cabbage,  which  demand  relatively 
less  care.  In  manuring  and  rotating  crops,  this  difference 


294 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


in  absorptive  power  must  be  considered,  not  only  to 
secure  the  maximum  effect  upon  the  crop  manured,  but 
also  to  get  the  greatest  residual  effect  of  the  manure 
upon  succeeding  crops. 


FIG.  100.  Deep  and  shallow  cultivation  for  corn.  On  the  right-hand  side  of 
the  picture  the  deep  cultivator  shovels  are  destroying  the  upper  roots.  On  the 
left-hand  side  the  shallow  cultivation  does  not  reach  the  roots. 

162.  Cereal  crops. — These  plants  possess  the  power  of 
utilizing  the  potassium  and  phosphorus  of  the  soil  to 
a  considerable  degree,  but  generally  require  fertili- 
zation with  nitrogen  salts.  Most  of  the  cereals,  like  wheat, 
rye,  oats  and  barley,  take  up  most  of  their  nitrogen 
early  in  the  season,  before  the  nitrification  processes 
have  been  sufficiently  operative  to  furnish  a  large  supply 


ABSORPTIVE  POWER  OF  CEREALS  295 

of  nitrogen,  and  hence  nitrogen  is  the  fertilizer  consti- 
tuent that  usually  gives  best  results,  and  should  be 
added  in  a  soluble  form.  Wheat,  in  particular,  needs 
a  large  amount  of  soluble  nitrogen  early  in  its  spring 
growth.  Since  it  is  a  delicate  feeder,  it  does  best  after  a 
cultivated  crop  or  a  fallow,  by  which  the  nitrogen  has 
been  converted  into  a  soluble  form.  Oats  can  make 
better  use  of  the  soil  fertility  and  does  not  require 
so  much  manuring.  Maize  is  a  very  coarse  feeder,  and, 
while  it  removes  a  very  large  quantity  of  plant-food 
from  the  soil,  it  does  not  require  that  these  be  added 
in  a  soluble  form.  Farm  manure  and  other  slowly  acting 
manures  may  well  be  applied  for  the  maize  crop.  The 
long  growing  period  required  by  the  maize  plant  gives 
it  opportunity  to  utilize  the  nitrogen  as  it  becomes 
available  during  the  summer,  when  ammonification  and 
nitrification  are  active.  Phosphorus  is  the  substance 
usually  most  needed  by  maize. 

163.  Grass  crops. — Grasses,  when  in  meadow  or  in 
pasture,  are  greatly  benefited  by  manures.  They  are 
less  vigorous  feeders  than  the  cereals,  have  shorter 
roots,  and,  when  left  down  for  more  than  one  year,  the 
lack  of  aeration  in  the  soil  causes  decomposition  to 
decrease.  There  is  usually  a  more  active  fixation  of 
nitrogen  in  grasslands  than  in  cultivated  lands,  but  this 
becomes  available  very  slowly. 

Different  soils  and  different  climatic  conditions 
necessitate  different  methods  of  manuring  for  grass. 
Farm  manures  may  well  be  applied  to  meadows  in  all 
situations,  while  the  use  of  nitrogen  is  generally  profi- 
table. 


296         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

154.  Leguminous    crops. — Most    of    the    leguminous 
crops  are  deep-rooted  and  are  vigorous  feeders.    Their 
ability  to  acquire  nitrogen  from  the  air  makes  the  use 
of  that  fertilizer  constituent  unnecessary  except  in  a 
few  instances,  such  as  young  alfalfa  on  poor  soil,  where 
a  small  application  of  nitrate  of  soda  is  usually  bene- 
ficial.   Lime   and   potassium   are  the  substances   most 
beneficial  to  legumes  on  the  majority  of  soils. 

155.  Root-crops. — Many    of    the    members    of    this 
class  of  crops  will  utilize  very  large  amounts  of  plant- 
food  if  it  is  in  a  form  in  which  they  can  use  it.    Phos- 
phates and  nitrogen   are  the  substances  generally  re- 
quired, the  latter  especially  by  beets  and  carrots. 

156.  Vegetables. — In  growing  vegetables,  the  object 
is  to  produce  a  rapid  growth  of  leaves  and  stalks  rather 
than  seeds,  and  often  this  growth  is  made  very  early  in 
the  season.   As  a  consequence,  a  soluble  form  of  nitrogen 
is   very  desirable.    Farm   manure  should  also  have  a 
prominent  part  of  the  treatment,  as  it  keeps  the  soil  in  a 
mechanical  condition  favorable  to  retention  of  moisture, 
which  vegetables  require  in  large  amounts,  and  it  also 
supplies  needed  fertility.    The  very  intensive  method 
of  culture  employed  in  the   production  of   vegetables 
necessitates    the    use    of    much    greater    quantities    of 
manures  than  are  used  for  field  crops,  and  the  great 
value  of  the  product  justifies  the  practice. 

157.  Fruits. — In  manuring  fruits,  with  the  exception 
of  some  of  the  small,  rapid-growing  ones,  it  is  the  aim 
to  maintain  a  continuous  supply  of  nutrients  available 
to  the  plant,  but  not  sufficient  for  stimulation,  except 
during  the  early  life  of  the  tree,  when  rapid  growth 


ABSORPTION   BY   SOIL  PARTICLES  297 

of  wood  is  desired.  An  acre  of  apple  trees  in  bearing 
removes  as  much  plant-food  from  the  soil  in  one  season 
as  does  an  acre  of  wheat. 

Farm  manure  and  a  complete  fertilizer  may  be  used, 
of  which  the  constituents  should  be  in  a  fairly  available 
form,  as  a  constant  supply  is  necessary. 

V.     ABSORPTION    BY    THE    SOIL    OF    SUBSTANCES 
IN    SOLUTION 

If  the  brown  water  extract  from  manure  be  filtered 
through  a  clay  soil  not  containing  soluble  alkalies,  the 
filtrate  will  be  nearly  colorless.  Many  solutions  of  dye 
stuffs  are  affected  in  the  same  way.  Solution  of  alkali  or 
alkaline  earth  salts  are  more  or  less  modified  by  this 
operation,  the  bases  being  retained  by  the  soil.  Thus 
when  a  solution  of  the  nitrate,  sulfate,  or  chloride  of  any 
one  of  these  bases  is  filtered  through  the  soil,  a  part  of 
the  base  is  absorbed  by  the  soil,  while  the  acid  comes 
through  in  the  filtrate.  If  these  bases  are  in  the  form 
of  phosphates  or  silicates,  not  only  the  base  is  absorbed 
but  the  acid  as  well. 

158.  Substitution  of  bases. — Associated  with  the 
absorption  of  the  base  from  solution,  there  is  liberation 
of  some  other  base  from  the  soil,  which  combines  with 
the  acid  in  the  solution  and  appears  in  the  filtrate  as  a 
salt  of  that  acid. 

When  absorption  takes  place  from  solution,  the  base 
is  never  entirely  removed,  no  matter  how  dilute  the 
solution  may  be.  A  dilute  solution  of  potassium  chloride 
filtered  through  a  soil  will  produce  a  filtrate  containing 


298 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


some  calcium  chloride  or  sodium  chloride,  or  both,  and 
some  potassium  chloride.  The  more  dilute  the  solution, 
the  larger  the  proportion  retained.  Peters  treated  100 
grams  of  soil  with  250  c.c.  of  a  solution  of  potassium 
salts,  and  found  that  the  potassium  of  different  salts  was 
retained  in  different  proportions,  and  that  the  stronger 
solutions  lost  relatively  less  than  the  weaker. 

TABLE  XLV 


i 

To  normal 

To  normal 

Grams  KjO 
absorbed 

Grams  KjO 
absorbed 

KCL  .. 

.3124 

.1990 

K2SO4  

.3362 

.2098 

K3CO«  

.5747 

.3134 

The  same  bases  are  not  always  absorbed  to  the  same 
extent  by  different  soils;  one  soil  may  have  a  greater 
absorptive  power  for  potassium,  while  another  may  retain 
more  ammonia.  They  seem  to  be  interchangeable,  as  any 
absorbed  base  may  be  released  by  another  in  solution. 

159.  Time  required  for  absorption. — The  amount  of 
absorption  depends  upon  the  time  of  contact  between  the 
soil  and  the  solution.  While  a  large  part  of  the  dis- 
solved base  is  taken  up  in  a  short  time  after  being  in 
contact  with  the  soil,  the  maximum  absorption  is  only 
effected  after  considerable  time.  Ammonia,  according 
to  Way,  reaches  its  maximum  absorption  in  half  an 
hour,  while  Henneberg  &  Stohmann  found  that  phos- 
phorus required  twenty-four  hours  to  reach  the  same 
degree  of  absorption. 


PROPERTIES  OF  ABSORBED  SUBSTANCES         299 

160.  Insolubility    of    certain    absorbed    substances. — 
Although  bases  once  absorbed  may  be  easily  displaced 
by  other  bases,  it  is  a  difficult  matter  to  dissolve  them 
from  the  soil  with  pure  water.   Peters  treated  100  grams 
of   soil   with    250   c.c.    of   water   containing   potassium 
chloride,  of  which  .2114  grams  of  K2O  were  absorbed. 
The  soil  was  then  leached  with  distilled  water,  using 
125  c.c.  of  water  daily  for  ten  days.    At  the  end  of  that 
time  .0875  grams  of  K2O  had  been  removed,  or  at  the 
rate  of  28,100  parts  of  water  to  one  part  of  K2O  dis- 
solved from  the  soil.    Henneberg  and  Stohmann  found 
that  it  required  10,000  parts  of  water  to  dissolve  one 
part  of  absorbed  ammonia  from  the  soil. 

161.  Influence    of    size    of    particles. — The    surface 
area  of    the  soil-particles    determines    to  some  extent 
the  amount  of  substance  absorbed.    For  this,  and  other 
reasons,  a  fine-grained  soil  absorbs  a  greater  quantity 
of  material  than  a  coarse-grained  soil.    In  fact,  it  was 
early  shown  by  Way  that  the  absorption  phenomenon 
is  largely  a  function  of  the  silt,  clay  and  humus  of  the 
soil. 

162.  Causes  of  absorption. — A  number  of  causes  have 
been  assigned  for  the  absorption  of  substances  by  soils, 
and  there  can  be  no  doubt  that  the  phenomenon  is  not 
due  to  any  one  process.   Several  distinct  causes  are  now 
quite  generally  recognized  and,  while  others  that  have 
been  suggested   may  have  a  part  in   the  result,   they 
cannot  all  be  taken  up  at  this  time.    The  better-known 
and  more  important  absorption  processes  are  the  fol- 
lowing: 

163.  Zeolites. — As     stated    on     a     preceding     page, 


300          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

Way  demonstrated  that  sand  had  little  absorbing  power 
as  compared  with  clay,  and  further,  that  when  the 
zoelitic  silicates  were  removed  from  clay  by  digestion 
with  hydrochloric  acid,  the  clay  largely  lost  its  power 
of  absorption.  Way  produced  an  artificial  hydrated 
silicate  of  alumina  and  soda,  and  Eichorn  found  natural 
hydrated  silicates  or  zeolites  that  removed  bases  with 
the  substitution  of  other  bases,  in  the  manner  of  natural 
soil.  A  further  characteristic  of  these  zeolites  is  that 
the  replaced  base  is  present  in  the  filtrate  in  amounts 
chemically  equivalent  to  the  base  removed. 

It  has  further  been  shown  that  the  absorptive  power 
of  soils  is  more  or  less  proportional  to  the  amount  of 
acid  soluble  silicates  it  contains.  The  zeolites  being 
rather  easily  soluble  in  strong  mineral  acids,  it  is  held 
that  the  bases  so  combined  are  more  readily  available 
to  plants  than  in  most  combinations  found  in  the  soil, 
and  yet  are  not  readily  leached  out  of  it. 

Soluble  bases  added  to  the  soil  in  manures  are  taken 
up  and  held  by  zeolites,  instead  of  being  removed  in  the 
drainage  water.  However,  nitric  acid,  important  as  it 
is  to  agriculture,  is  not  absorbed,  and,  together  with  the 
sulfuric  and  hydrochloric  acid,  is  quickly  but  not  com- 
pletely removed  from  the  soil  by  drainage  water. 

164.  Other  absorbents. — Humus,  ferric  and  alumi- 
num hydrates,  and  calcium  carbonate,  exercise  absor- 
bent properties,  but  to  what  extent  and  of  what  import- 
ance it  is  difficult  to  say.  Soils  rich  in  humus,  without 
doubt,  owe  much  of  their  fertility  to  the  retention  by 
that  constituent  of  a  large  supply  of  readily  available 
plant-food  material.  Many  prairie  soils  that  have  been 


ADSORPTION  301 

reduced  in  productiveness  under  cultivation  respond 
to  the  application  of  organic  matter  in  a  remarkable 
manner.  Humus  in  these  soils  seems  to  be  the  chief 
conserver  of  readily  available  plant-food  materials. 

Ferric  and  aluminum  hydrate  aid  in  the  retention 
of  acids,  notably  phosphoric,  by  forming  highly  in- 
soluble compounds. 

166.  Adsorption. — There  is  a  physical  absorption, 
termed  adsorption,  due  to  the  concentration  of  the  soil 
solution  in  contact  with  the  surface  of  the  particles. 
The  phenomenon  is  familiarly  exemplified  in  the  clari- 
fying effect  of  the  charcoal  filter.  This  process  results 
in  the  retention  of  considerable  soluble  material  in  fine- 
grained soils,  that  would  otherwise  be  washed  out. 
In  the  case  of  nitrates,  which  are  not  retained  by  the 
zeolites,  adsorption  is  an  important  factor.  (See  page 
325.)  If  a  solution  of  a  known  quantity  of  nitrate  of 
soda  be  added  to  a  clay  soil,  and  it  is  then  attempted  to 
extract  the  nitrate  from  the  soil  with  distilled  water, 
it  will  be  found  impossible  to  recover  a  very  appreciable 
per  cent  of  the  amount  added.  While  adsorption  prob- 
ably does  not  account  for  all  of  the  nitrates  retained, 
there  can  be  no  doubt  that  it  plays  an  important  part. 
Nutritive  salts  held  in  this  way  are  readily  available 
to  the  plant  whose  root-hairs  come  in  contact  with  the 
soil  particles. 

166.  Occlusion. — According  to  Wiley,  clay  in  a  col- 
loidal state  has  the  property  of  dissociating  to  a  certain 
extent  potash  salts,  and  entangling  the  basic  ion  in  the 
meshes  of  the  colloid  structure.  How  extensive  or 
important  this  action  is  has  not  been  demonstrated. 


302 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


167.  Absorption  as  related  to  drainage. — The  drainage 
water  from  cultivated  fields  in  the  humid  region,  and 
to  a  less  extent  in  the  semi-arid  and  arid  region,  except 
where  irrigation  is  practiced,  carries  off  very  consider- 
able amounts  of  plant-food  material.  The  loss  of  this 
material  is  due  to  the  operation  of  the  various  natural 
disintegrating  agents  upon  the  soil  mass,  and  to  the 


FIG.  101.     Wasting  manure  by  leaching. 


application  of  fertilizing  materials  in  a  soluble  form. 
The  various  absorptive  properties  stand  between  the 
natural  solubility  of  the  soil  and  the  tendency  to  loss  in 
drainage,  and  hold  these  materials  that  would  otherwise 
be  lost,  in  a  condition  in  which  they  may  readily  be  used 
by  the  plant. 

168.  Substances  usually  carried  in  drainage  water.— 
However,  some  material  is  always  lost  in  drainage 
water,  of  which,  among  the  bases  of  the  soil  those  most 


SUBSTANCES   REMOVED   IN  DRAINAGE   WATER    303 

likely  to  be  found  are  soda,  magnesia  and  lime,  and  of 
the  acids  nitric,  carbonic,  hydrochloric  and  sulfuric. 
Nitric  acid  and  lime  undergo  the  most  serious  losses. 
The  former  may  be  curtailed  to  a  great  extent  by  keeping 
crops  growing  on  the  soil,  during  all  of  the  time  that 
nitrification  is  going  on,  and  if  the  crop  does  not  mature 
or  if,  for  any  other  reason,  it  is  not  desired  to  harvest 
the  crop,  it  should  be  plowed  under,  to  return  the  nitro- 
gen in  the  form  of  organic  matter.  A  crop  used  for  this 
purpose  is  called  a  "catch  crop."  Rye  is  used  quite 
commonly  as  a  catch  crop,  as  it  continues  growth  until 
late  in  the  fall,  and  resumes  growth  early  in  the  spring, 
conserving  nitrates  whenever  nitrification  may  occur, 
and  it  may  then  be  plowed  under  to  prepare  the  land 
for  another  crop.  Rye  also  has  the  advantage  of  small 
cost  for  seed. 

The  loss  of  calcium  cannot  well  be  prevented,  and 
the  use  of  commercial  fertilizers  always  greatly  in- 
creases such  loss.  The  only  remedy  is  the  application 
of  some  form  of  calcium  to  the  soil. 

169.  Drainage  records  at  Rothamsted. — Drainage 
water  from  a  series  of  plats  at  the  Rothamsted  Experi- 
ment Station,  which  have  been  manured  in  various 
ways  and  planted  to  wheat  each  year  since  1852,  have 
been  analysed  at  certain  times,  and  the  results  of  these 
analyses,  as  compiled  by  Hall,  give  some  idea  of  the  loss 
of  salts  from  cultivated  soils.  The  drainage  water  was 
obtained  from  the  tile  drains,  one  of  which  extended 
under  each  plat  from  one  end  to  the  other,  and  opened 
into  a  ditch,  so  that  the  water  could  be  collected  when 
desired.  The  analyses  shown  in  the  accompanying  table 


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(304) 


SUBSTANCES  REMOVED  IN   DRAINAGE   WATER    305 

were  made  by  Dr.  A.  Voelcker,  and  represent  the  mean 
of  not  more  than  five  collections  made  in  December, 
May  and  January  and  April  during  a  period  of  two  years. 
They  can  not  be  regarded  as  showing  accurately  the 
annual  removal  of  salts  from  the  soil  but  are  still  sig- 
nificant. 

From  this  table  it  will  be  seen  that  lime  is  the  in- 
gredient lost  in  largest  amount  from  this  soil  and  that 
the  character  of  the  manure  applied  influences  this  loss 
to  some  extent.  The  sulfates  of  sodium,  potassium, 
and  magnesium  have  notably  increased  the  loss  of  lime, 
as  have  also  the  ammonium  salts.  The  loss  of  lime  from 
all  of  the  manured  plats  was  notably  greater  than  from 
the  unmanured. 

Potash  was  not  removed  in  large  amount  by  the 
drainage  water  from  any  of  the  plats.  Ammonium 
salts  with  superphosphate  and  with  magnesia  occasioned 
only  a  slight  loss  of  potash,  as  did  also  the  absence  of 
manure.  The  plats  receiving  mineral  manure  alone  and 
farm  manure  lost  the  greatest  quantities  of  potash. 

The  quantity  of  sulfuric  acid  leached  from  the  soil 
is  quite  large  and  highly  variable.  It  is  frequently, 
but  by  no  means  uniformly,  large  on  those  plats  from 
which  lime  is  removed  in  large  amounts.  The  plat 
receiving  farm  manure  lost  the  largest  quantity  of  sul- 
furic acid. 

Phosphoric  acid  was  removed  in  "small  amounts  and, 
except  in  the  case  of  the  unmanured  plat,  those  plats 
losing  the  least  phosphoric  acid  gave  the  largest  yields. 
The  loss  of  phosphoric  acid  seems  to  be  a  matter  of 
failure  on  the  part  of  the  crop  to  utilize  it.  rather  than 
its  liberation  bv  anv  manurial  substance. 


306         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

Ammoniacal  nitrogen  in  the  drainage  water  is  very 
small  in  amount,  but  nitrate  nitrogen  is  present  in 
amounts  sufficient  to  make  the  loss  of  some  concern. 
The  use  of  sodium  nitrate  occasioned  the  greatest  loss 
of  nitrogen  while  ammonium  salts  and  farm  manure 
contributed  nearly  as  much.  Forty  to  fifty  pounds  of 
nitrogen  per  acre  may  be  lost  annually  in  this  way, 
which  amount  would  have  a  commercial  value  of  from 
eight  to  nine  dollars. 

The  most  serious  losses  are  those  of  nitrogen  and 
lime,  and  both  are  to  an  extent  unavoidable.  Potassium 
and  phosphorus,  which  must  also  be  purchased  in  ma- 
nures, are  lost  only  at  the  rate  of  a  few  pounds  per  acre 
but  had  lime  been  applied  to  any  of  these  plats,  the  loss 
of  potassium  would  probably  have  been  larger.  Nitro- 
gen and  phosphorus  are  best  conserved  by  keeping 
crops  growing  on  land  as  much  of  the  time  as  possible, 
and  the  former  may  also  be  protected  by  applying  the 
soluble  nitrogen  salts  only  at  a  time  when  they  can  be 
utilized  by  crops.  The  loss  of  calcium  frequently  amounts 
to  several  hundred  pounds  per  acre  annually,  and,  as 
the  presence  of  calcium  carbonate  is  essential  to  a 
healthy  condition,  of  the  soil  this  loss,  particularly 
from  the  soil  receiving  salts  like  sulfates  and  chlorides, 
the  bases  of  which  are  absorbed  by  plants  in  larger 
amounts  than  the  acids,  is  likely  to  result  in  a  very  bad 
condition  of  the  soil.  The  only  method  of  obviating  this 
is  to  lime  the  soil  from  time  to  time. 

170  Relation  of  absorptive  capacity  to  productive- 
ness.— The  absorptive  capacity  of  a  soil  is  not  so  much 
a  measure  of  its  immediate  as  of  its  permanent  produc- 


ALKALI  SOILS  307 

tiveness.  It  is  well  known  that  a  very  sandy  soil  responds 
quickly  to  the  application  of  soluble  manures,  but  that 
the  effect  is  confined  mainly  to  one  season;  while  a  clay 
soil,  although  not  so  quickly  responsive  to  fertilization, 
shows  the  effect  of  the  application  much  more  markedly 
the  second  or  third  year  than  does  the  sandy  soil.  Me- 
chanical absorption  holds  the  nutritive  material  in  a  very 
readily  available  condition,  while  absorption  by  zeo- 
litic  bodies  renders  these  substances,  somewhat  less 
readily  available.  There  are  also  other  reasons  why  the 
sandy  soil  is  more  responsive. 

It  cannot  be  said  that  there  is  a  relation  between  the 
absorptive  capacity  of  a  soil  and  its  productiveness  when 
manured  or  when  nearly  virgin,  but  soil  long-cultivated 
and  unmanured  frequently  show  such  a  relation.  King, 
in  working  with  eight  types  of  soil  in  different  portions 
of  the  United  States,  found  that  those  soils  removing 
the  most  potassium  from  solution  gave  the  largest  yield 
of  crop.  It  would  not  be  permissible,  however,  to  adopt 
this  test  as  a  method  for  determining  productiveness 
in  soils. 

VI.     ALKALI    SOILS 

As  already  explained  (page  14),  soils  are  acted  upon 
by  a  great  variety  of  agencies,  which  gradually  render 
soluble  a  portion  of  the  particles.  The  soluble  matter 
is  taken  up  by  the  soil  water,  and  in  humid  regions 
where  a  large  amount  of  water  percolates  through  the 
soil  and  passes  off  in  the  drainage,  the  soluble  matter 
is  found  only  in  small  quantity  at  any  time.  In  arid 
regions  the  loss  by  drainage  is  slight  or  entirely  wanting, 


308 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


and  under  such  conditions  the  soluble  materials  accumu- 
late in  the  soil,  being  transposed  downward  with  the 
percolating  water  and  upward  again  with  the  capillary 
rise  of  water  during  the  dry  period.  The  lower  soil  may 
at  one  time  contain  considerably  more  soluble  salt 
than  the  upper  soil,  while  at  another  time  the  upper 


Fio.  102.     Bare  spot,  marking  the  first  appearance  of  injurious  quantities  of 
alkali  salts  in  the  surface  layer  of  soil.   Utah. 

soil  may  contain  more  of  these  salts,  in  which  case  the 
solution  in  contact  with  plant-roots  may,  and  often 
does,  contain  so  much  soluble  matter  that  vegetation 
is  injured  or  destroyed.  This  excess  of  soluble  salts  may 
or  may  not  have  a  marked  alkaline  reaction,  but  in 
any  case  produce  what  are  termed  alkali  soils. 


KINDS   OF   ALKALI  309 

171.  Composition    of    alkali    salts. — The    materials 
dissolved  in  the  soil  water  consist  of  all  of  the  sub- 
stances found  in  the  soil,  but,  as  the  rates  of  solubility 
of   these   substances   vary   greatly,    there   accumulates 
a   much   larger  quantity   of  some  substances   than   of 
others.    Carbonates,  sulfates  and  chlorides  of  sodium, 
potassium,  calcium  and  magnesium  occur  in  the  largest 
amounts.   Sodium  may  be  present  as  carbonate,  sulfate, 
chloride,    phosphate   and   nitrate.     Potassium    may   be 
similarly  combined.    Magnesium  is  likely  to  appear  as  a 
sulfate  or  chloride,  and  calcium  as  a  sulfate,  chloride 
or  carbonate.    In  some  soils  one  salt  will  predominate, 
and  in  other  soils  other  salts  will  prevail.   A  base  may  be 
present    in    combination    with    several    different    acids. 
The  nature  of  the  prevailing  salt  influences  greatly  the 
effect  upon  vegetation.   Table  XLVII  gives  the  composi- 
tion of  the  soluble  salts  from  a  number  of  alkali  soils. 

172.  White  and  black  alkali. — Sulfates  and  chlorides 
of  the  alkalies  when  concentrated  on  the  surface  of  the 
soil  produce  a  white  incrustation,  which  is  very  common 
in  alkali  regions  during  a  dry  period,  as  a  result  of  evapo- 
ration of  moisture.    Alkali  in  which  these  acids  predomi- 
nate is  called  white  alkali. 

Carbonates  of  the  alkalies  dissolve  organic  matter 
in  the  soil,  thus  giving  a  dark  color  to  the  solution 
and  to  the  incrustation,  and  for  this  reason  alkali  con- 
taining large  quantities  of  these  salts  is  called  black 
alkali. 

Black  alkali  is  much  more  destructive  to  vegetation 
than  is  white.  A  quantity  of  white  alkali  that  would 
not  seriously  interfere  with  the  growth  of  most  crops 


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312 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


might  completely  prevent  the  growth  of  useful  crops 
if  the  alkali  were  black. 

173.  Effect  of  alkali  on  crops. — The  presence  of  rela- 
tively large  amounts  of  salts  dissolved  in  water  and 
brought  in  contact  with  a  plant  cell  has  been  shown  by 
DeVries  to  cause  a  shrinking  of  the  protoplasmic  lining 
of  the  cell,  the  shrinking  increasing  with  the  concentration 


!  -p> 


Fro.  103.  Showing  plasmolysis  of  plant  cells  produced  by  strong  solutions 
of  salts,  (a)  Normal  cells;  (6)  cell  subjected  to  action  of  5  per  cent  solution 
of  KNOj,  showing  (z)  cell-wall,  (p1,  p2)  plasmatic  membranes,  (a)  vacuole; 
(c)  cell  subjected  to  action  of  2.3  per  cent  solution  of  KNO3,  causing  a  slight 
contraction  of  the  plasmatic  membranes. 

of  the  solution.  This  causes  the  plant  to  wilt,  cease 
growth  and  finally  die.  The  nature  of  the  salt,  and  the 
species  and  even  the  individuality  of  the  plant,  deter- 
mine the  point  of  concentration  at  which  the  plant 
succumbs. 

174.  Direct  effect. — The  directly  injurious  effect  of 
the  chlorides,  sulfates,  nitrates,  etc.,  of  the  alkalies 
and  alkali  earths  is  due  to  this  action  on  the  cell  con- 
tents. The  carbonates  of  the  alkalies  have,  in  addition, 
a  corroding  effect  upon  the  plant  tissues,  dissolving  the 
portions  of  the  plant  with  which  they  come  in  contact. 


EFFECTS   OF  ALKALI  ON   CROPS  313 

175.  Indirect  effect. — Indirectly  alkali  salts  may  in- 
jure plants  by  their  influence  upon  the  soil  tilth,  soil 
organisms,  and  fungous  and  bacterial  diseases. 

176.  Effect  upon  different  crops. — The  factors  that 
determine  the  tolerance  of  plants  to  alkali  are:  (1)  The 
physiological  constitution  of  the  plant.    (2)  The  rooting 
habit. 

The  first  is  not  well  understood,  but  resistance  varies 
with  species,  and  even  with  individuals  of  the  same 
species.  So  far  as  the  rooting  habit  influences  tolerance 
of  alkali,  the  advantage  is  with  the  deep-rooted  plants 
like  alfalfa  and  sugar-beets,  probably  because  at  least 
a  part  of  the  root  is  in  a  less  strongly  impregnated  portion 
of  the  soil. 

Of  the  cereals,  barley  and  oats  are  the  most  tolerant, 
being  able  in  some  cases  to  produce  a  fair  crop  on  soil 
containing  one-tenth  per  cent  of  white  alkali.  Of  the 
forage  crops,  a  number  of  valuable  grasses  are  able  to 
grow  with  somewhat  more  than  one-tenth  per  cent  of 
alkali.  Timothy,  smooth  brome  and  alfalfa  are  the  cul- 
tivated forage  plants  most  tolerant  of  alkali, — although 
they  do  not  equal  the  native  grasses  in  this  respect. 
Cotton  will  also  tolerate  a  considerable  amount  of  alkali. 

177.  Other  conditions  influencing  the  action  of  alkali. 
—The  larger  the  water  content  of  the  soil,  the  less  the 

injury  to  plants  from  alkali;  but,  should  the  same  soil 
become  dry,  the  previous  large  quantity  of  water  would, 
by  bringing  into  solution  a  larger  amount  of  alkali, 
render  the  solution  stronger  than  it  would  otherwise  have 
been,  and  thus  cause  more  injury. 

The  distribution  of  the  alkali  at  different  depths  may 


314         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

have  an  important  bearing  on  its  effect  upon  plants. 
Young  plants  and  shallow-rooted  plants  may  be  entirely 
destroyed  by  the  concentration  of  alkali  at  the  surface, 
when  the  same  quantity  evenly  distributed  through  the 
soil,  or  carried  by  moisture  to  a  lower  depth,  would  have 
caused  no  difficulty. 

A  loam  soil,  by  reason  of  its  greater  water-holding 
capacity,  will  carry  more  alkali  without  injury  to  plants 
than  will  a  sandy  one. 

Certain  of  the  alkali  salts  exert  a  deflocculating 
action  upon  clay  soils,  and  effect  an  indirect  injury  in 
that  way. 

178.  Reclamation  of  alkali  land. — The  alkali  salts, 
being  readily  soluble,  are  carried  by  the  soil-water  where 
there  is   any   lateral   movement,   as  frequently   occurs 
where  land  slopes  to  some  one  point.    Low-lying  lands 
adjacent  to  such  slopes  are  thus  likely  to  contain  con- 
siderable  alkali,    and  the   ''alkali   spots"   of  semi-arid 
regions  and  the  lar^e  accumulations  of  alkali  in  many 
of  the  valley  lands  of  arid  regions  are  traceable  to  this 
cause. 

179.  Irrigation    and    alkali. — In    irrigated    regions, 
the  injurious  effect   of  alkali  is  frequently  discovered 
only  after  irrigation  has  been  practised  for  a  few  years. 
This  is  due  to  what  is  known  as  a  "rise  of  alkali,"  and 
comes  about  through  the  accumulation,  near  the  surface 
of   the   soil,    of   salts   that    were   formerly    distributed 
throughout  a  depth  of  perhaps  many  feet.    Before  the 
land  was  irrigated,  the  rainfall  penetrated  only  a  slight 
depth  into  the  soil,  and  when  evaporation  took  place 
salts  were  drawn  to  the  surface  from  only  a  small  volume 


RECLAMATION   OF   ALKALI   LAND  315 

of  soil.  When,  however,  irrigation  water  was  turned 
upon  the  land,  the  soil  became  wet  for  perhaps  fifteen 
or  twenty  feet  in  depth.  During  the  portion  of  the  year 
in  which  the  soil  is  allowed  to  dry,  large  quantities  of 
salts  are  carried  to  the  upper  soil  by  the  upward-moving 
capillary  water.  These  salts  are  in  part  carried  down  again 
by  the  next  irrigation,  but  the  upward  movement  con- 
stantly exceeds  the  downward  one.  This  is  because  the 
descending  water  passes  largely  through  the  non-capillary 
interstitial  spaces,  while  the  ascending  water  passes  en- 
tirely through  the  capillary  ones.  The  smaller  spaces, 
therefore,  contain  quite  a  quantity  of  soluble  salt  after 
the  downward  movement  ceases  and  the  upward  move- 
ment commences.  In  other  words,  the  volume  of  water 
carrying  downward  the  salts  in  the  capillary  spaces  is  less 
than  that  carrying  them  upward  through  these  spaces. 
Surface  tension  causes  the  salts  to  accumulate  largely  in 
the  capillary  spaces,  and  it  is  therefore  the  direction  of 
the  principal  movement  through  these  that  determines 
the  point  of  accumulation  of  the  alkali. 

There  are  large  areas  of  land  in  Egypt.  India  and  even 
in  France  and  Italy,  as  well  as  in  this  country,  that  have 
suffered  in  this  way,  and  not  infrequently  they  have 
reverted  to  a  desert  state. 

There  are  a  number  of  methods  that  have  been  used 
with  more  or  less  success  to  reclaim  alkali  land. 

180.  Underdrainage. — -Of  the  various  mot  hods  for 
removing  an  excess  of  soluble  salts  the  use  of  tile  drains 
is  the  most  thorough  and  satisfactory.  When  this  is 
used  in  an  irrigated  region,  heavy  and  repeated  appli- 
cations of  water  must  be  made,  to  leach  out  the  alkali 


316          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

from  the  soil  and  drain  it  off  through  the  tile.  When 
used  for  the  amelioration  of  alkali  spots  in  a  semi-arid 
region,  the  natural  rainfall  will  in  .time  effect  the  removal. 

In  laying  tiles,  it  is  necessary  to  have  them  at  such 
a  depth  that  soluble  salts  in  the  soil  beneath  them  will 
not  readily  rise  to  the  surface.  This  will  depend  upon 
those  properties  of  the  soil  governing  the  capillary  move- 
ment of  water.  Three  or  four  feet  frequently  suffices, 
but  the  capillary  movement  should  first  be  determined. 

After  drains  have  been  placed,  the  land  is  flooded  with 
water  to  a  depth  of  three  or  four  inches.  This  is  allowed 
to  soak  into  the  soil  and  pass  off  through  the  drains, 
leaching  out  part  of  the  alkali  in  the  process.  Before  the 
soil  has  time  to  become  very  dry  the  flooding  is  repeated 
and  the  operation  kept  up  until  the  land  is  brought  into 
a  satisfactory  condition. 

Crops  that  will  stand  flooding  may  be  grown  during 
this  treatment,  and  they  will  serve  to  keep  the  soil  from 
puddling,  as  it  is  likely  to  do  if  allowed  to  dry  on  the 
surface.  If  crops  are  not  grown,  the  soil  should  be  har- 
rowed between  floodings. 

The  operation  should  not  be  carried  to  a  point  where 
the  soluble  salts  are  reduced  below  the  needs  of  the 
crop,  or  to  lose  entirely  their  effect  upoa  the  retention  of 
moisture. 

181.  Correction  of  black  alkali. — The  use  of  gypsum 
on  black-alkali  land  has  sometimes  been  practiced  for 
the  purpose  of  converting  the  alkali  carbonates  into  sul- 
fates,  thus  ameliorating  the  injurious  properties  of  the 
alkali  without  decreasing  the  amount.  The  quantity 
of  gypsum  required  may  be  calculated  from  the  amount 


NEUTRALIZATION  OF  BLACK  ALKALI 


317 


and  composition  of  the  alkali.  The  soil  must  be  kept 
moist,  in  order  to  bring  about  the  reaction,  and  the 
gypsum  should  be  harrowed  into  the  surface,  and  not 
plowed  under. 


Fia.  104.    hromits  inrrmi.i  growing  on  rcrhiimml  alkali  l;uui. 

When  soil  containing  black  alkali  is  to  be  tile-drained, 
it  is  recommended  that  the  land  first  be  treated  with 
gypsum,  as  the  substitution  of  alkali  sulfates  for  carbo- 
nates causes  the  soil  to  assume  a  much  less  compact 
condition  and  thus  facilitates  drainage,  as  well  as  pre- 
venting the  loss  of  organic  matter  dissolved  by  the  alkali 


318         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

carbonates,  and  soluble  phosphates,  both  of  which  are 
precipitated  by  the  change. 

182.  Retarding     evaporation. — As     evaporation     of 
moisture  from  the  surface  of  the  soil  is  the  cause  of  rise 
of  alkali,  it  is  important  to  reduce  evaporation  to  a 
minimum,    either    in    drained    or    in    undrained    land. 
Especially  where  irrigation  is  practiced  without  drainage, 
it  becomes  desirable  to  use  as  little  water  as  is  necessary 
to  produce  good  crops,  and  to  conserve  this  to  the  utmost 
by  checking  evaporation  from  the  surface  of  the  soil. 

The  methods  used  for  checking  evaporation  are  the 
maintenance  of  a  soil  or  other  mulch,  and  of  a  good 
tilth.  (See  page  195.)  In  handling  alkali  spots  in  the 
semi-arid  region,  it  is  very  important  to  reduce  evapo- 
ration to  the  smallest  amount  practicable. 

183.  Cropping  with  tolerant  plants. — Certain  alkali 
soils  that  are  strongly  impregnated  with  alkali  may  be 
gradually  improved  by  cropping  with  sugar-beets  and 
other  crops  that  are  tolerant  of  alkali,  and  which  re- 
move large  amounts  of  salts.    This  is  more  likely  to  be 
efficacious  where  irrigation  is  not  practiced.    . 

184.  Other  methods. — Numerous  other   methods  of 
disposing  of  alkali  or  ameliorating  its  effects  have  been 
used    or    proposed.     Among    these    are    the    following: 
(1)  "Leaching,"  which  consists  of  flooding  the  surface 
of  the  soil  for  the  purpose  of  carrying  the  soluble  salts 
down  to  a  depth  of  three  or  four  feet,  where  they  will 
not  effect  the  roots  of  ordinary  crops.    If  natural  drain- 
age exists,  this  plan  is  effective  and  without  danger; 
otherwise  evaporation  must  be  reduced  to  the  smallest 
possible  amount.    (2)  Removal  of  alkali  by  scraping  the 


MANURES          v  319 

surface  when  the  salts  have  accumulated  there  in  time 
of  drought.  While  this  may  aid  in  the  work  of  ameliora- 
tion, it  is  not  a  final  solution  of  the  difficulty.  (3)  Wash- 
ing the  alkali  from  the  land  by  turning  on  a  rapidly 
moving  body  of  water,  when  the  alkali  is  encrusted  on 
the  surface  of  the  soil,  has  been  tried,  but  with  poor  suc- 
cess, as  the  alkali  is  largely  carried  into  the  soil,  instead 
of  being  removed  by  the  water  passing  over  the  surface 
of  the  land. 

185.  Alkali  spots. — In  semi-arid  regions,  small  areas 
of  alkali  are  frequently  found,  varying  from  a  few  square 
yards  to  several  acres  in  size.  The  quantities  of  alkali 
in  these  are  usually  not  sufficient  to  prevent  the  growth 
of  crops  in  years  of  good  rainfall,  but  in  periods  of 
drought  the  concentration  of  the  salts  and  the  compact 
condition  they  tend  to  produce  combine  to  injure  the 
crop.  The  methods  already  mentioned  for  treating  alkali 
land  are  of  service  on  these  small  areas,  and,  in  addition, 
the  plowing  under  of  fresh  farm  manure  has  been  found 
to  improve  their  productiveness.  This,  with  surface 
drainage,  deep  tillage  and  good  cultivation,  to  prevent 
the  soil  from  drying  out,  will  usually  remedy  the  diffi- 
culty. Frequently  these  spots  become  highly  productive 
under  proper  treatment. 

VII.     MANURES 

A  manure  is  any  solid  substance  added  to  the  soil  to 
make  it  more  productive.  This  it  may  do:  (1)  By  im- 
proving the  physical  condition  of  the  soil,  as  usually 
results  from  the  application  of  lime  and  the  incorporation 


320         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

of  organic  matter.  (2)  By  favoring  the  action  of  useful 
bacteria,  which  is  one  of  the  most  beneficial  results  of 
farm  manure,  and  also  of  lime.  (3)  By  counteracting 
the  effects  of  toxic  substances,  as,  for  instance,  the  con- 
version of  sodium  carbonate  into  sulfate  by  gypsum, 
or  the  neutralization  of  acidity,  or  possibly  the  removal 
of  toxic  organic  substances  by  certain  salts.  (4)  By 
adding  to  the  soil  the  nutrient  materials  absorbed  by 
plants,  which  results  in  the  case  of  almost  all  substances 
used  as  manures. 

186.  Early    ideas    of    the    function    of    manures.— 
Manures  were  at  one  time  supposed  to  pulverize  the  soil, 
and  the  French  word    manoeuvrer,  from  which  the  word 
manure  comes,  means  to  work  with  the  hand.   This  idea 
probably  originated  through  the  observation  that  farm 
manure,  which  was  the  only  manure  in  use  at  that  time, 
made  the  soil  less  cloddy. 

It  has  been  argued,  notably  by  Jethro  Tull,  that  as 
tillage  pulverizes  the  soil  it  may  be  used  as  a  substitute 
for  manures.  There  are,  however,  conditions  aside  from 
tilth  that  are  influenced  by  manures,  and  good  tilth 
alone  will  not  suffice  to  maintain  a  permanently  intensive 
agriculture.  It  is  true  in  the  United  States,  as  it  is  in 
Europe,  that  a  large  consumption  of  manures  goes  hand- 
in-hand  with  a  highly  developed  and  intensive  system  of 
farming. 

187.  Development  of  the  idea  of  nutrient  function  of 
manures. — While  the  use  of  animal  excrement  on  cul- 
tivated soils  was  practiced  as  far  back  as  systematic 
agriculture  can  be  definitely  traced,  the  earliest  record 
of  the  use  of  mineral  salte  for  increasing  the  yield  of 


HISTORY   OF   COMMERCIAL  FERTILIZERS         321 

crops  was  published,  in  1669,  by  Sir  Kenelm  Digby.  He 
says,  "  By  the  help  of  plain  salt  petre,  diluted  in  water, 
and  mingled  with  some  other  fit  earthly  substance, 
that  may  familiarize  it  a  little  with  the  corn  into  which 
I  endeavored  to  introduce  it,  I  have  made  the  barrenest 
ground  far  outgo  the  richest  in  giving  a  prodigiously 
plentiful  harvest."  His  dissertation  does  not,  however, 
show  any  true  conception  of  the  reason  for  the  increase 
in  the  crop  through  the  use  of  this  fertilizer.  In  fact, 
the  want  of  any  real  knowledge  at  that  time  of  the  com- 
position of  the  plant  would  have  made  this  impossible. 

In  1804,  Theodore  de  Sausure  published  his  chemical 
researches  upon  plants,  in  which  he,  for  the  first  time, 
called  attention  to  the  significance  of  the  ash  ingredients 
of  plants,  and  pointed  out  that  without  them  plant-life 
is  impossible,  and  further,  that  only  the  ash  of  the  plant 
tissue  is  derived  from  the  soil. 

Justus  von  Liebig,  in  his  writings  published  about 
1840,  emphasized  still  more  strongly  the  importance 
of  mineral  matter  in  the  plant,  and  its  extraction  from 
the  soil.  He  refuted  the  theory,  at  that  time  popular, 
that  plants  absorb  their  carbon  from  humus,  but  made 
the  mistake  of  attaching  little  importance  to  the  pres- 
ence of  humus  in  the  soil.  He  showed  the  importance 
of  potassium  and  phosphorus  in  manures,  but,  in  his 
later  expressions,  failed  to  appreciate  the  value  of 
nitrogenous  manures,  holding  that  a  sufficient  amount 
is  washed  from  the  atmosphere  in  the  form  of  ammonia. 

A  true  conception  of  the  necessity  for  a  supply  of 
combined  nitrogen  in  the  soil  was  even  at  that  time  enter- 
tained by  Boussingault  and  by  Sir  John  Lawes,  although 


322         THE   PRINCIPLES   OF  SOIL  MANAGEMENT 

the  elaborate  experiments  conducted  by  Lawes,  Gilbert 
and  Pugh,  in  1857,  were  required  to  fully  demonstrate 
the  fact.  Their  care  in  conducting  the  experiments 
resulted  in  their  sterilizing  the  soil  with  which  they 
experimented,  and  hence  their  failure  to  discover  the 
utilization  of  free  atmospheric  nitrogen  by  legumes. 

Between  1840  and  1850,  Sir  John  Lawes  began  the 
manufacture  of  bone  superphosphate,  and,  about  the 
same  time,  Peruvian  guano  and  nitrate  of  soda  were 
introduced  into  Europe.  The  commerical  fertilizer 
industry  thus  dates  from  this  time. 

188.  Classes  of  manures. — While  manures  are  very 
numerous  as  to  kind,  and  a  certain  manure  may  have 
a  number  of  distinct  functions,  they  may  yet  be  roughly 
divided  into  classes.   They  will  accordingly  be  treated 
under  the  following  heads:  (1)  Commercial  fertilizers. 
(2)  Farm  manures.   (3)  Green  manures.    (4)  Soil  amend- 
ments. 

189.  Commercial  fertilizers. — Although  the  commer- 
cial fertilizer  industry  is  little  more  than  half  a  century 
old,  the  sale  of  fertilizers  in  this  country  amounts  to 
about  $50,000,000  annually.    Animal  refuse  and  phos- 
phate fertilizers  are  exported,  while  nitrate  of  soda  and 
potassium  salts  are  imported. 

Of  the  fertilizers  sold  in  1899,  about  70  per  cent  was 
consumed  in  the  North  Atlantic  and  South  Atlantic 
states,  in  an  area  lying  within  300  miles  of  the  seaboard. 
Nearly  one-half  of  the  remainder  was  purchased  in  four 
states,  Ohio,  Indiana,  Alabama  and  Louisiana. 

190.  Function  of  commercial  fertilizers. — Primarily 
the  function  of  commercial  fertilizers  is  to  add  plant 


324         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

nutrients  to  the  soil,  usually  in  a  form  more  readily 
soluble  than  those  already  present  in  large  quantity. 
While  other  beneficial  effects  may  be  produced  by 
certain  fertilizers,  they  are  usually  of  secondary  import- 
ance, as  compared  with  the  addition  of  the  plant  nutri- 
ents. 

191.  Fertilizer  constituents. — Prepared  fertilizers,  as 
found  on  the  market,  are  usually  composed  of  a  number 
of  ingredients.   As  these  are  the  carriers  of  the  fertilizing 
material,  and  as  it  is  upon  their  composition  and  solu- 
bility that  the  value  of  the  fertilizer  depends,  a  knowl- 
edge of  the  properties  of  these  constituents  is  of  interest 
to  every  user  of  fertilizers,  and  is  a  valuable  aid  in  their 
purchase. 

192.  Fertilizers   used   for   their   nitrogen. — Nitrogen 
is  the  most  expensive  constituent  of  manures,  and  is  of 
great  importance,  as  it  is  very  likely  to  be  deficient  in 
soils.     A   commercial   fertilizer  may   have  its  nitrogen 
in  the  form  of  soluble  inorganic  salt,  or  combined  as 
organic   material.    Upon  the  form  of  combination  de- 
pends to  a  certain  extent  the  value  of  the  nitrogen, 
as  the  soluble  inorganic  salts  are  very  readily  available 
to  the  plant,  while  the  organic  forms  must  pass  through 
the  various  processes  leading  to  nitrification  before  the 
plant  can  use  the  nitrogen  so  contained.    The  inorganic 
nitrogen  fertilizers  are  sodium  nitrate,  ammonium  sul- 
fate,  calcium  nitrate  and  calcium  cyanamid. 

193.  Sodium  nitrate. — This  fertilizer  now  constitutes 
the  principal  source  of  inorganic  nitrogen  in  commercial 
fertilizers.    The  salt  occurs  in  the  crude  condition  in 
Northern  Chili,  and  is  believed  to  be  due  to  the  action 


NITROGEN   BEARING   FERTILIZERS  325 

of  soil  organisms  acting  through  a  very  long  period,  and 
leaving  the  product  finally  in  the  form  of  sodium  nitrate 
that  has  crystalized  out  of  solution  in  which  it  has  some- 
time been  held.  The  crude  salt  is  purified  by  crystalli- 
zation, and,  as  put  upon  the  market,  contains  about 
9(>  per  cent  sodium  nitrate,  or  about  16  per  cent  of  nitro- 
gen, 2  per  cent  of  water,  and  small  amounts  of  chlorides, 
sulfates  and  insoluble  matter.  The  cost  of  nitrogen  in 
this  form  is  from  fifteen  to  eighteen  cents  per  pound. 

On  account  of  its  easy  availability,  sodium  nitrate 
acts  quickly  in  inducing  growth.  For  this  reason  it  is 
used  much  by  market  gardeners,  and  for  other  purposes 
when  a  rapid  growth  is  desired.  It  is  the  most  active 
form  of  nitrogen.  A  light  dressing  on  meadow  land  in  the 
early  spring  assists  greatly  in  hastening  growth  by  fur- 
nishing available  nitrogen  before  the  conditions  are 
favorable  for  the  process  of  nitrification.  On  small 
grain  it  serves  a  similarly  useful  purpose  where  the  soil 
is  not  rich. 

Owing  to  the  fact  that  it  is  not  absorbed  by  the  soil 
in  large  quantities,  it  is  easily  lost  in  the  drainage  water; 
for  which  reason  it  should  only  be  applied  when  crops 
are  growing  upon  the  soil,  and  then  only  in  moderate 
quantity. 

The  continued  and  abundant  use  of  sodium  nitrate 
upon  the  soil  may  result,  through  its  deflocculating 
action,  in  breaking  down  aggregates  of  soil-particles, 
thus  compacting  and  injuring  the  structure.  This  effect 
is  attributed  to  the  accumulation  of  sodium  salts,  par- 
ticularly the  carbonate,  as  the  sodium  is  not  utilized 
by  the  plant  to  the  same  extent  as  is  the  nitrogen. 


326         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

194.  Ammonium  sulfate. — When  coal  is  distilled, 
a  portion  of  the  nitrogen  is  liberated  as  ammonia,  and  is 
collected  by  passing  the  products  of  distillation  through 
water  in  which  the  ammonia  is  soluble,  forming  the 
ammoniacal  liquor.  The  ammonia  thus  held  is  distilled 
into  sulfuric  acid  with  the  formation  of  ammonium 
sulfate  and  the  removal  of  impure  gases. 

Commercial  ammonium  sulfate  contains  about  20  per 
cent  of  nitrogen.  It  is  the  most  concentrated  form  in 
which  nitrogen  can  be  purchased  as  a  fertilizer,  having 
from  sixty  to  eighty  pounds  more  of  nitrogen  per  ton 
than  sodium  nitrate.  It  is,  therefore,  economical  to 
handle.  Its  effect  upon  crops  is  not  so  rapid  as  that  of 
sodium  nitrate,  but  it  is  not  so  quickly  carried  from  the 
soil  by  drainage  water,  as  the  ammonium  salts  are 
readily  absorbed  by  the  soil.  A  pound  of  nitrogen  in  the 
form  of  sulfate  has  about  the  same  value  as  the  same 
amount  in  the  form  of  nitrate. 

The  long  and  extensive  use  of  ammonium  sulfate  on 
a  soil  has  a  tendency  to  produce  an  acid  condition, 
through  the  accumulation  of  sulfates  which  are  not 
largely  taken  up  by  plants. 

Ammonium  sulfate,  like  sodium  nitrate,  should 
not  be  applied  in  the  autumn,  as  the  ammonia  is  con- 
verted into  nitrates  and  leached  from  the  soil  in  sufficient 
quantities  to  entail  a  very  decided  loss  of  nitrogen. 
There  is  not  likely  to  be  so  large  a  loss  of  nitrogen  from 
ammonium  salts  as  from  nitrates,  and,  as  would  naturally 
be  expected,  there  is  greater  loss  of  nitrogen  when  these 
salts  are  used  alone  than  when  they  are  combined  with 
other  fertilizing  ingredients. 


LOSS   OF  SOIL   NITROGEN 


327 


Hall  has  estimated  the  loss  of  nitrogen  from  certain 
drained  plats,  of  the  Rothamsted  Experiment  Station. 
This  estimate  is  based  upon  the  concentration  of  the 
drainage  from  the  different  plats,  of  which  there  was 
no  record  of  total  flow,  but  for  which  the  measurements 
of  flow  from  the  lysimeter  draining  60  inches  of  soil 
were  taken,  and  the  total  loss  of  nitrates  calculated 
on  this  basis.  Estimated  in  this  way,  the  effects  of  sev- 
eral different  methods  of  manuring  are  shown  in  the 
accompanying  table. 

TABLE  XLVIII 
POUNDS  PER  ACRE  NITRIC  NITROGEN  IN  DRAINAGE  WATER 


Treatment 

1879-80 

1880-81 

Spring 
sowing 
to 
harvest 

Harvest 
to 
spring 
sowing 

Spring 
sowing 
to 
harvest 

Harvest 
to 
spring 
sowing 

Unmanured        

1.7 
1.6 
18.3 
45.0 

9.6 
42.9 

19.0 

10.8 
13.3 
12.6 
15.6 

59.9 
14.3 

16.4 

4.7 

0.6 

0.7 
4.3 
15.0 

3.4 

7.4 

3.7 
1.8 

17.1 

17.7 
21.4 
41.0 

74.9 
35.2 

2.->.3 
18.8 

Mineral  fertilizers  only     

Minerals  4-  400  pounds  ammon.  salts.  . 
Minerals  +  550  pounds  nitrate  of  soda  . 
Minerals   +   400  pounds  ammon.  salts 
applied  in  autumn  
400  pounds  ammon.  salts  alone 

400  pounds  ammon.  salts  +  sulfate  of 
potash                               .    .        .    . 

Estimated  drainage  in  inches  

11.1 

This  table,  in  addition  to  confirming  the  statements 
already  made  in  regard  to  the  loss  of  nitrogen  in  drain- 
age waters,  also  shows  how  closely  the  supply  of  avail- 
able nitrogen  was  used  by  the  crops  on  those  plats, 


328         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

evidently  in  need  of  nitrogen  fertilization,  as  these  plats 
lost  very  little  nitrogen  during  the  growing  season, 
while  during  the  remainder  of  the  year  they  lost  nearly 
as  much  as  did  some  of  the  nitrogen  manured  plats. 
It  also  indicates  that  the  loss  when  nitrate  is  used  is 
greater  than  when  ammonium  salts  are  applied,  as  the 
amount  of  nitrogen  in  the  550  pounds  of  nitrate  is  really 
eight  pounds  per  acre  more  than  in  the  400  pounds 
ammonium  sulfate,  which  is  not  sufficient  to  account 
for  the  difference  in  the  loss.  However,  half  of  the 
nitrate  treated  plat  received  no  other  manure,  and 
produced  only  a  small  crop,  which  would  naturally 
result  in  a  a  greater  loss  by  drainage. 

195.  Calcium  cyanamid. — The  vast  store  of  atmos- 
pheric nitrogen  chemically  uncombined,  but  very  inert, 
will  furnish  an  inexhaustible  supply  of  this  highly  valu- 
able fertilizing  element,  when  it  can  be,  with  reasonable 
economy,  combined  in  some  manner  that  will  result 
in  a  product  commercially  transportable,  and  that  will, 
when  placed  in  the  soil,  be  or  become  soluble  without 
liberating  substances  toxic  to  plants.  The  importance 
of  the  nitrogen  supply  for  agriculture  may  be  appreciated 
when  we  consider  that  nitrates  are  being  carried  off 
in  the  drainage  water  of  all  cultivated  soils  at  the  rate 
of  from  twenty-five  to  fifty  pounds  and  even  more  per 
acre,  annually,  and  that  nearly  as  much  more  is  removed 
in  crops. 

The  exhaustion  of  the  supply  of  nitrogen  in  most 
soils  may  be  accomplished  within  one  or  two  generations 
of  men,  unless  a  renewal  of  the  supply  be  brought  about 
in  some  way.  Natural  processes  provide  for  an  annual 


LIME   NITROGEN  329 

accretion  through  the  washing  down  of  ammonia  and 
nitrates  by  rain-water  from  the  atmosphere,  and  through 
the  fixation  of  free  atmospheric  nitrogen  by  bacteria; 
but,  without  the  frequent  use  of  leguminous  crops, 
the  supply  could  not  be  maintained.  Farm  practice 
of  the  present  day  requires  the  application  of  nitrogen 
in  some  form  of  manure,  and,  as  the  end  of  the  commer- 
cial supply  of  combined  nitrogen  is  easily  in  sight,  there 
is  urgent  need  of  discovering  a  new  source.  This  has 
lately  been  done  by  combining  calcium  with  atmospheric 
nitrogen  in  the  forms  of  calcium  cyanamid  and  cal- 
cium nitrate. 

The  most  successful  process  for  the  production  of 
cyanamid  consists  in  passing  nitrogen  into  closed  retorts 
containing  powdered  calcium  carbide  heated  to  a  tem- 
perature of  1,100°  C.,  the  product  being  calcium  cyana- 
mid, and  free  carbon. 


,  +  2N=CaCNJ-l-C. 

The  free  carbon  remains  distributed  in  the  cyanamid 
and  gives  it  a  black  color.  A  modification  of  the  process 
provides  for  the  use  of  lime  and  coke  instead  of  calcium 
carbide,  but  this  has  not  yet  been  used  on  a  commercial 
scale.  The  nitrogen  required  for  the  process  is  obtained 
either  by  passing  air  over  heated  copper,  or  by  the  frac- 
tional distillation  of  liquid  air. 

The  fertilizer,  as  placed  on  the  market,  is  a  heavy, 
black  powder  with  a  somewhat  disagreeable  odor.  At 
present  it  is  not  manufactured  in  America  and  is  not 
obtainable  except  in  small  amounts.  Plants  for  its 
production  are  being  promoted,  which  will  doubtless7 


330          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

result  in  its  being  placed  on  the  market  in  the  near 
future. 

There  are,  at  present,  two  calcium  cyanamid  ferti- 
lizers being  manufactured.  One  is  called  lime-nitrogen, 
and  is  made  in  Italy;  the  other  is  called  nitrogen-lime, 
and  is  made  in  the  province  of  Saxony,  Germany.  The 
former  contains  15  to  23  per  cent  nitrogen,  40  to  42 
per  cent  calcium,  and  17  to  18  per  cent  carbon  dust. 
The  latter  is  said  to  contain  somewhat  less  nitrogen, 
and  to  have  in  it  some  calcium  chloride,  which  is  some- 
times injurious  to  plants. 

The  value  of  calcium  cyanamid  as  a  fertilizer  has  not 
yet  been  definitely  and  conclusively  ascertained.  The 
cyanamid  must  be  decomposed  before  becoming  avail- 
able to  the  plant.  Under  favorable  conditions,  the  nitro- 
gen of  the  cyanamid  is  converted  into  ammonia;  but, 
if  the  conditions  for  decomposition  are  not  favorable, 
the  dicyanamid  may  be  formed,  which  has  a  poisonous 
effect  upon  plants.  Another  objection  which  sometimes 
obtains  is  that  acetylene  is  produced  from  the  carbide, 
which  remains  unchanged  in  the  manufacture  of  the 
cyanamid.  Acetylene  is  also  injurious  to  plants. 

By  incorporating  the  calcium  cyanamid  in  the  soil 
eight  to  fourteen  days  before  the  seed  is  planted,  this 
difficulty  may  be  overcome.  It  is  also  important  that 
the  cyanamid  be  plowed  under,  and  not  left  on  or  near 
the  surface  of  the  soil,  as,  under  these  circumstances, 
decomposition  does  not  go  on  properly,  and  the  poisonous 
action  above  referred  to  takes  place. 

Upon  heavy  soil  the  value  of  cyanamid  as  a  fertilizer 
is  not  greatly  below  that  of  sodium  nitrate,  but  upon 


CALCIUM   NITRATE  331 

sandy  soil  it  ranks  much  lower.  Indeed,  it  appears  to  be 
but  poorly  suited  to  use  on  sandy  soils. 

196.  Calcium  nitrate.  —  The  other  process  for  com- 
bining atmospheric  nitrogen  is  of  even  more  recent 
invention  than  that  for  the  manufacture  of  calcium 
cyanamid  and,  like  it,  is  not  conducted  on  a  commercial 
scale  in  this  country;  but,  with  the  vast  opportunities 
for  developing  electric  power  which  are  offered  in  certain 
localities,  factories  for  the  manufacture  of  calcium  nitrate 
will  soon  be  established. 

The  process  employs  an  electric  arc  to  produce  nitric 
oxide  by  the  combustion  of  atmospheric  nitrogen,  ac- 
cording to  the  simple  equation: 


A  very  high  power  is  required  for  this  synthesis, 
involving  a  temperature  of  2,500°  to  3,000°  C.,  and  the 
expense  of  the  operation  is  determined  almost  entirely 
by  the  cost  of  the  electricity. 

The  nitric  oxido  gas  is  passed  through  milk  of  lime, 
giving  calcium  nitrate. 

The  calcium  nitrate  produced  by  this  process  has  a 
yellowish  white  color,  and  is  easily  soluble  in  water, 
but  deliquesces  very  rapidly  in  the  air.  This  last  prop- 
erty can  be  overcome  by  adding  an  excess  of  lime  in  the 
manufacture,  thus  producing  a  basic  calcium  nitrate, 
which  contains  only  8.9  per  cent  nitrogen.  Another 
way  of  avoiding  the  difficulties  involved  by  the  deliques- 
cent property  of  the  nitrate  is  practiced  by  the  factory 
at  Nottoden,  Norway.  This  consists  in  first  melting 
the  product,  then  grinding  it  fine,  and  packing  it  in 


332         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

air-tight  casks.    The  fertilizer  thus  prepared  contains 

11  to  13  per  cent  nitrogen. 

Calcium  nitrate  contains  its  nitrogen  in  a  form 
directly  available  to  plants.  It  resembles  sodium  nitrate 
in  its  solubility,  availability,  and  lack  of  absorption  by 
the  soil.  It  may  be  spread  upon  the  surface  of  the  ground, 
as  it  exerts  no  poisonous  action,  and  does  not  tend  to 
form  a  crust,  as  does  sodium  nitrate. 

The  relative  values  of  the  different  soluble  nitrogen 
fertilizers  vary  with  a  great  many  conditions  and  can 
be  accurately  judged  only  by  a  large  number  of  tests. 
At  present,  both  the  calcium  nitrate  and  the  cyanamid 
are  being  produced  at  less  cost  per  pound  of  nitrogen 
than  is  sodium  nitrate,  when  laid  down  in  the  neighbor- 
hood of  the  factories  in  Europe.  It  seems  quite  certain 
that,  when  the  processes  have  been  further  improved, 
the  result  will  be  to  greatly  reduce  the  cost  of  the  avail- 
able nitrogen. 

197.  Organic  nitrogen  in  fertilizers. — The  commercial 
fertilizers  containing  organic  nitrogen  include  cotton- 
seed-meal, which  contains  7  per  cent  nitrogen,  when 
free  from  hulls;  linseed-meal,  with  5.5  per  cent  nitrogen; 
castor  pomace,  having  6  per  cent  nitrogen;  and  a  number 
of  refuse  products  from  packing-houses,  among  which 
there  are  red-dried  blood  and  black-dried  blood,  the 
former  having  about  13  per  cent  nitrogen,  and  the  latter 
6  to  12  per  cent;  dried  meat  and  hoof  meal,  carrying 

12  to  13  per  cent  nitrogen;  ground  fish  containing  8  per 
cent  nitrogen;  and  tankage,  of  which  the  concentrated 
product  has  a  nitrogen  content  of  10  to  12  p'er  cent,  and 
the  crushed  tankage,  4  to  9  per  cent;  also  leather-meal 


OTHER    FORMS   OF   ORGANIC   NITROGEN  333 

and  wool-and-hair  waste,  which  last  two,  on  account 
of  their  mechanical  condition,  are  of  practically  no  value. 

The  meals  made  from  seeds  are  primarily  stock-foods, 
but  are  sometimes  used  as  manures.  They  decompose 
rather  slowly  in  the  soil,  owing  to  their  high  oil  content, 
and  are  much  more  profitably  fed  to  live  stock  than 
applied  as  farm  manure.  They  contain  some  phosphorus 
as  well  as  nitrogen. 

.  Guano  consists  of  the  excrement  and  carcasses  of 
sea-fowl.  The  composition  of  guano  depends  upon  the 
climate  of  the  region  in  which  it  is  found.  Guano  from 
an  arid  region  contains  nitrogen,  phosphorus  and  potas- 
sium, while  that  from  a  region  where  rains  occur  con- 
tains only  phosphorus — the  nitrogen  and  potassium 
having  been  leached  out.  In  a  dry  guano  the  nitrogen 
occurs  as  uric  acid,  urates,  and,  in  small  quantities, 
as  ammonium  salts.  A  damp  guano  contains  more 
ammonia.  The  phosphorus  is  present  as  calcium  phos- 
phate, ammonium  phosphate,  and  as  the  phosphates  of 
other  alkalies.  A  portion  of  the  phosphate  is  readily 
soluble  in  water.  All  of  the  plant-food  is  thus  either 
directly  soluble,  or  becomes  so  soon  after  admixture 
with  the  soil.  The  composition  is  extremely  variable. 
The  best  Peruvian  guano  contains  from  10  to  12  per  cent 
of  nitrogen,  12  to  15  per  cent  phosphoric  acid,  and  3  to  4 
per  cent  of  potash. 

Guano  was  formerly  a  very  important  fertilizing 
material,  but  the  supply  has  become  so  nearly  exhausted 
that  it  is  relatively  unimportant  at  the  present  time. 

Of  the  abattoir  products,  dried  blood  is  the  most 
readily  decomposed,  and  therefore  has  its  nitrogen 


334         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

in  the  most  available  form.  In  fact,  it  produces  results 
more  quickly  than  any  other  form  of  organic  nitrogen. 
It  requires  a  condition  of  soil  favorable  to  decomposi- 
tion and  nitrification,  which  prevents  its  exerting  a 
strong  action  in  the  early  spring.  It  should  be  applied 
to  the  soil  before  the  crop  is  planted.  The  black  dried 
blood  contains  from  2  to  4  per  cent  of  phosphoric  acid. 

Dried  meat  contains  a  high  percentage  of  nitrogen, 
but  does  not  decompose  so  easily,  and  is  not  so  desirable 
a  form  of  nitrogen.  It  can  be  fed  to  hogs  or  poultry  to 
advantage,  and  the  resulting  manure  is  very  high  in 
nitrogen. 

Hoof-meal,  while  high  in  nitrogen,  decomposes  slowly, 
being  less  active  than  dried  blood.  It  is  of  use  in  in- 
creasing the  store  of  nitrogen  in  a  depleted  soil. 

Ground  fish  is  an  excellent  form  of  nitrogen,  and  is  as 
readily  available  as  blood,  but  has  a  lower  nitrogen 
content. 

Tankage  is  highly  variable  in  composition,  and  the 
concentrated  tankage,  being  more  finely  ground,  under- 
goes more  readily  the  decomposition  necessary  for  the 
utilization  of  the  nitrogen.  Crushed  tankage  contains 
from  3  to  12  per  cent  of  phosphoric  acid,  in  addition 
to  its  nitrogen. 

Leather-meal  and  wool-and-hair  waste  are  in  such 
a  tough  and  undecomposable  condition  that  they  may 
remain  in  the  soil  for  years  without  losing  their  struc- 
ture. They  are  not  to  be  recommended  as  manures. 

198.  Fertilizers  used  for  their  phosphorus. — Phos- 
phorus is  generally  present  in  combination  with  lime, 
iron  or  alumina.  Some  of  the  phosphates  also  contain 


PHOSPHATE   FERTILIZERS  335 

organic  matter,  in  which  case  they  generally  carry  some 
nitrogen.  Phosphates  associated  with  organic  matter 
decompose  more  quickly  in  the  soil  than  untreated 
mineral  phosphates. 

199.  Bone  phosphate. — Formerly,   bones   were  used 
entirely   in    the   raw    condition,    ground   or   unground. 
When  ground,  they  are  a  more  quickly  acting  fertilizer 
than    when   unground.     Raw   bones   contain    about   22 
per  cent  phosphoric  acid  and  4  per  cent  nitrogen.    The 
phosphorus  is  in  the  form  of  tricalcic  phosphate  (Ca3 
(P04)a). 

Most  of  the  bone  now  on  the  market  is  first  boiled  or 
steamed,  which  frees  it  from  fat  and  nitrogenous  matter, 
both  of  which  are  used  in  other  ways.  Steamed  bone 
is  a  more  valuable  fertilizer  than  raw  bone,  as  the  fat 
in  the  latter  retards  decomposition,  and  also  because 
steamed  bone  is  in  a  better  mechanical  condition.  The 
form  of  the  phosphoric  acid  is  the  same  as  in  raw  bone, 
and  constitutes  28  to  30  per  cent  of  the  product,  while 
the  nitrogen  is  reduced  to  1 J  per  cent. 

Bone  tankage,  which  has  already  been  spoken  of  as  a 
nitrogenous  fertilizer,  contains  from  7  to  9  per  cent 
phosphoric  acid,  largely  in  the  form  of  tricalcium  phos- 
phate. All  of  these  bone  phosphates  are  slow-acting 
manures,  and  should  be  used  in  a  finely  ground  form, 
arid  for  the  permanent  benefit  of  the  soil  rather  than  as 
an  immediate  source  of  nitrogen  or  phosphorus. 

200.  Mineral   phosphates.  —There  are   many   natural 
deposits  of  mineral  phosphates  in  different   portions  of 
the  world,  some  of  the  most  important  of  which  are  in 
North  America.   The  phosphorus  in  all  of  these  is  in  the 


336         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

form  of  tricalcium  phosphate,  but  the  materials  asso- 
ciated with  it  vary  greatly. 

Apatite  is  found  in  large  quantities  in  the  provinces 
of  Ontario  and  Quebec,  Canada.  It  occurs  chiefly  in 
crystalline  form. 

The  tricalcium  phosphate  of  which  it  is  composed 
is  in  one  form  associated  with  calcium  fluoride,  and 
in  the  other  with  calcium  chloride.  The  Canadian 
apatite  contains  about  40  per  cent  phosphoric  acid, 
being  richer  than  that  found  elsewhere.  Phosphorite 
is  another  name  for  apatite,  but  is  chiefly  applied  to 
the  impure  amorphous  form. 

Caprolites  are  concretionary  nodules  found  in  the 
chalk  or  other  deposits  in  the  south  of  England,  and  in 
France.  They  contain  25  to  30  per  cent  of  phosphoric 
acid,  the  other  constituents  being  calcium  carbonate  and 
silica. 

South  Carolina  phosphate  contains  from  26  to  28  per 
cent  of  phosphoric  acid,  and  but  a  very  small  amount 
of  iron  and  alumina.  As  these  substances  interfere  with 
the  manufacture  of  superphosphate  from  rock,  their 
presence  is  very  undesirable, — rock  containing  more  than 
from  3  to  6  per  cent  being  unsuitable  for  that  purpose. 

Florida  phosphates  occur  in  the  form  of  soft  phos- 
phate, pebble  phosphate,  and  boulder  phosphate. 
Soft  phosphate  contains  from  18  to  30  per  cent  of  phos- 
phoric acid,  and,  on  account  of  its  being  more  easily 
ground  than  most  of  these  rocks,  is  often  applied  to 
the  land  without  being  first  converted  into  a  superphos- 
phate. The  other  two,  pebble  phosphate  and  boulder 
phosphate,  are  highly  variable  in  composition,  ranging 


SUPERPHOSPHATE  337 

from  20  to  40  per  cent  phosphoric  acid.  Tennessee  phos- 
phate contains  from  30  to  35  per  cent  of  phosphoric 
acid. 

Basic  slag,  or,  as  it  is  also  called,  phosphate  slag  or 
Thomas  phosphate,  is  a  by-product  in  the  manufacture 
of  steel  from  pig-iron  rich  in  phosphorus.  The  phos- 
phorus present  is  in  the  form  of  tetracalcium  phosphate, 
(CaO)4P2O5.  It  also  contains  calcium,  magnesium, 
aluminum,  iron,  manganese  silica  and  sulfur.  On  ac- 
count of  the  presence  of  iron  and  aluminum,  and  because 
its  phosphorus  is  more  readily  soluble  than  the  trical- 
cium  phosphate,  the  ground  slag  is  applied  directly 
to  the  soil  without  treatment  with  acid. 

201.  Superphosphate  fertilizers.  —  In  order  to  render 
more  readily  available  to  plants  the  phosphorus  con- 
tained in  bone  and  mineral  phosphates,  the  raw  material, 
purified  by  being  washed  and  finely  ground,  is  treated 
with  sulfuric  acid.  This  results  in  a  replacement  of  phos- 
phoric acid  by  sulfuric  acid,  with  the  formation  of 
monocalcium  phosphate  and  calcium  sulfate,  and  a 
smaller  amount  of  dicalcium  phosphate,  according  to  the 
reactions: 


Ca3  (P04),  +  2  H,S04  =  CaH4  (PO4)a  +  2  CaSO.  ami 
('a, 


The  tricalcium  phosphate  being  in  excess  of  the  sul- 
furic acid  used,  a  part  of  it  remains  unchanged. 

In  the  treatment  of  phosphate  rock,  part  of  the 
sulfuric  acid  is  consumed  in  acting  upon  the  impurities 
present,  which  usually  consist  of  calcium  and  magnesium 
carbonates,  iron  and  aluminum  phosphates,  and  cal- 


338          THE   PRINCIPLES    OF   SOIL   MANAGEMENT 

cium  chloride  or  fluoride,  converting  the  bases  into  sul- 
fates  and  freeing  carbon  dioxide,  water,  hydrochloric 
acid  and  hydrofluoric  acid.  The  resulting  superphos- 
phate is  therefore  a  mixture  of  monocalcium  phosphate, 
dicalcium  phosphate,  tricalcium  phosphate,  calcium 
sulfate,  and  iron  and  aluminum  sulfates. 

In  the  superphosphates  made  from  bone,  the  iron 
and  aluminum  sulfates  do  not  exist  in  any  considerable 
amounts.  However,  as  long  as  the  phosphorus  remains 
in  the  form  of  monocalcium  phosphate,  the  value  of  a 
pound  of  available  phosphorus  in  the  two  kinds  of  fer- 
tilizer is  the  same;  but  the  remaining  tricalcium  phos- 
phate has  a  greater  value  in  the  bone  than  in  the  rock 
superphosphate. 

The  superphosphates  made  from  animal  bone  con- 
tain about  12  per  cent  available  phosphoric  acid,  and 
3  or  4  per  cent  of  insoluble  phosphoric  acid.  They  also 
contain  some  nitrogen.  Bone-ash  and  bone-black  super- 
phosphates contain  practically  all  of  their  phosphorus 
in  an  available  form,  but  they  contain  little  or  no  nitro- 
gen. South  Carolina  rock  superphosphate  contains  from 
12  to  14  per  cent  available  phosphoric  acid,  including 
from  1  to  3  per  cent  reverted  phosphoric  acid.  The  best 
Florida  rock  superphosphates  contain  from  17  per  cent 
downward  of  available  phosphoric  acid,  part  of  which 
is  reverted.  The  Tennessee  superphosphates  vary  from 
14  to  18  per  cent  available  phosphoric  acid. 

202.  Reverted  phosphoric  acid. — On  standing,  a 
change  sometimes  occurs  in  superphosphates  by  which 
a  part  of  the  phosphoric  acid  becomes  less  easily  soluble, 
and  to  that  extent  the  value  of  the  fertilizer  is  decreased. 


FORMS   OF  PHOSPHATE  339 

This  change,  known  as  "reversion,"  is  much  more  likely 
to  occur  in  superphosphates  made  from  rock  than  in 
those  derived  from  bone.  It  will  also  vary  in  different 
samples, — a  well-made  article  usually  undergoing  little 
^change,  even  after  long  standing.  It  is  supposed  to  be 
caused  by  the  presence  of  undecomposed  tricalcium 
phosphate,  and  of  iron  and  aluminum  sulfates. 

203.  Double    superphosphates. — In    making    super- 
phosphates, a  material  rich  in  phosphorus  must  be  used, 
— not  less  than  60  per  cent  tricalcium  phosphate  being 
necessary  for  their  profitable  production.    The  poorer 
materials  are  sometimes  used  in  making  what  is  known 
as  double  superphosphates.    For  this  purpose  they  are 
treated  with  an  excess  of  dilute  sulfuric  acid;  the  dis- 
solved phosphorus  and  the  excess  of  sulfuric   acid  are 
separated    from    the    mass   by    filtering   and    are   then 
used  for  treating  phosphates  rich  in  tricalcium  phosphate 
and    forming    superphosphates.     The    superphosphates 
so  formed  contain  more  than  twice  as  much  phosphorus 
as  those  made  in  the  ordinary  way. 

204.  Relative  availability  of  phosphate  fertilizers.— 
Superphosphates   and   double   superphosphates   contain 
their  phosphorus  in  a  form  in   which   it   can   lie  taken 
up  by  the  plant  at  once.   They  are  therefore  best  applied 
at  the  time  when  the  crop  is  planted,  or  shortly  before, 
or  they  may  be  applied  when  the  crop  is  growing.   Crude 
phosphates,  on  the  other  hand,  become  available  only 
through  the  natural  processes  in  the  soil.    The  presence 
of  decomposing  organic    matter  is   a   great    aid   to   the 
decomposition  of  crude  phosphates. 

Reverted  phosphorus,  although  not  soluble  in  water, 


340          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

is  readily  soluble  in  dilute  acids.  It  is  now  quite  gener- 
ally believed  that  it  furnishes  an  available  supply  of 
phosphorus  to  the  plant.  In  a  statement  of  fertilizer 
analyses  it  is  termed  "citrate  soluble,"  and  this  and  the 
"water  soluble"  are  termed  "available." 

The  degree  of  fineness  to  which  the  material  is  ground 
makes  a  great  difference  in  the  availability  of  the  less- 
soluble  phosphate  fertilizers,  especially  in  the  ground- 
rock  phosphates,  and  in  ground  bone.  This  material 
should  be  ground  fine  enough  to  pass  through  a  sieve 
having  meshes  one-fiftieth  of  an  inch  in  diameter. 

205.  Fertilizers  used  for  their  potassium. — The  pro- 
duction of  potassium  fertilizers  is  largely  confined  to 
Germany,  where  there  are  extensive  beds  varying  from 
50  to   150  feet  in  thickness,   lying  under  a  region  of 
country   extending  from   the   Harz    mountains   to   the 
Elbe  river,  and  known  as  the  Stassfurt  deposits.    De- 
posits have  lately  been   discovered  in  other  parts  of 
Germany. 

206.  Stassfurt    salts. — The    Stassfurt    salts    contain 
their  potassium  either  as  a  chloride  or  a  sulfate.    The 
chloride  has  the  advantage  of  being  more  diffusible  in 
the  soil,  but  in  most  respects  the  sulfate  is  preferable. 
Potassium  chloride  has  an  injurious  action  on  certain 
crops,  among  which  are  tobacco,  sugar-beets  and  pota- 
toes.    On   cereals,    legumes    and   grasses,    the    muriate 
appears  to  have  no  injurious  effect. 

The  mineral  produced  in  largest  quantities  by  the 
Stassfurt  mines  is  kainit.  Chemically  it  consists  of  mag- 
nesium and  potassium  sulfate,  and  magnesium  chloride, 
or  magnesium  sulfate  and  potassium  chloride.  Kainit 


POTASH-BEARING   FERTILIZERS  341 

has  the  same  action  on  plants  as  has  potassium  chloride. 
It  contains  from  12  to  20  per  cent  of  potash,  and  25  to 
45  per  cent  of  sodium  chloride,  with  some  chloride  and 
sulfate  of  magnesium. 

Kainit  should  be  applied  to  the  soil  a  considerable 
time  before  the  crop  for  which  it  is  intended  is  planted. 
It  should  not  be  drilled  in  with  the  seed,  as  the  action 
of  the  chlorides  in  direct  contact  with  the  seed  may 
injure  its  viability.  In  addition  to  the  potassium  added 
to  the  soil  by  kainit,  there  are  also  in  this  fertilizer 
magnesium  and  sodium.  The  magnesium  may  be  objec- 
tionable if  there  is  much  already  present  in  the  soil. 
(See  page  350.)  Sodium  may  to  some  extent  replace 
potassium  in  the  soil  economy,  and  in  that  way  may  be 
beneficial. 

Silvinit  contains  its  potassium  both  as  chloride  and 
as  sulfate.  It  also  contains  sodium  and  magnesium 
chlorides.  Potash  constitutes  about  1(>  per  cent  of  the 
material.  Owing  to  the  presence  of  chlorides,  it  has  the 
same  effect  on  plants  as  has  kainit. 

The  commerical  form  of  potassium  chloride  generally 
contains  about  80  per  cent  potassium  chloride,  or  50 
per  cent  potash.  The  impurities  are  largely  sodium 
chloride  and  insoluble  mineral  matter.  The  possible 
injury  to  certain  crops  from  the  use  of  the  chloride  has 
already  been  mentioned.  For  crops  not  so  affected, 
potassium  chloride  is  a  quick-acting  and  effective  carrier 
of  potassium,  and  one  of  the  cheapest  forms. 

High-grade  sulfate  of  potassium  contains  from  49 
to  51  per  cent  of  potash.  Unlike  the  muriate,  it  is  not 
injurious  to  crops  but  is  more  expensive. 


342        THE   PRINCIPLES    OF   SOIL    MANAGEMENT 

There  are  a  number  of  other  Stassfurt  salts,  consisting 
of  mixtures  of  potassium,  sodium  and  magnesium  in 
the  form  of  chlorides  and  sulfates.  They  are  not  so 
widely  used  for  fertilizers  as  are  those  mentioned  above. 

207.  Wood  ashes. — For  some  time  after  the  use  of 
fertilizers  became  an   important    farm    practice,   wood 
ashes  constituted  a  large  portion  of  the  supply  of  potas- 
sium.  They  also  contain  a  considerable  quantity  of  lime 
and  a  small  amount  of  phosphorus.   The  product  known 
as  unleached  wood  ashes  contains  5  to  6  per  cent  of  pot- 
ash, 2  per  cent  of  phosphoric  acid,  and  30  per  cent  of 
lime.   Leached  wood  ashes  contain  about  one  per  cent  of 
potash,  1^  per  cent  of  phosphoric  acid,  and  28  to  29  per 
cent  of  lime.  They  contain  the  potassium  in  the  form  of  a 
carbonate,  which  is  alkaline  in  its  reaction,  and  may  be 
injurious   to   seeds   when   in   large   amount.     They  are 
beneficial  to  acid  soils  through  the  action  of  both  the 
potassium  and  calcium  salts.  The  lime  is  valu-able  for  the 
other  effects  it  has  on  the  properties  of  the  soil.    (See 
page  348.) 

208.  Insoluble       potassium       fertilizers. — Insoluble 
forms  of  potassium,  occurring  in  many  rocks,  usually 
in  the  form  of  a  silicate,  are  not  regarded  as  having 
any  manurial  value.    Experiments  with  finely  ground 
feldspar  have  been  conducted  by  a  number  of  experi- 
menters, but  have,  in  the  main,  given  little  encourage- 
ment for  the  successful  use  of  this  material.   An  insoluble 
form  of  potassium  is  not  given  any  value  in  the  rating 
of  a  fertilizer,  based  upon  the  results  of  its  analysis. 

209.  Fertilizer  practice. — The   purchase   and  use  of 
commercial    fertilizers    is    an    art    that    requires    some 


BRANDS   OF   FERTILIZERS  343 

technical  knowledge  for  its  efficient  conduct.  There  are 
many  fertilizing  materials  put  up  under  numerous 
brands  that  must  be  selected  from  and  applied  to  a  great 
variety  of  crops  grown  on  innumerable  types  of  soil. 
The  result  is.  that  an  economical  fertilizer  practice  is 
difficult  to  establish,  and  the  use  of  fertilizers  is  usually 
conducted  in  an  entirely  empirical  manner. 

210.  Brands  of  fertilizers. — Each  manufacturer  or 
compounder  of  commercial  fertilizers  places  on  the 
market  a  number  of  brands  of  fertilizers  that  have  some 
trade  name,  frequently  implying  the  usefulness  of  the 
fertilizer  for  some  particular  crop,  but  without  reference 
to  the  character  of  the  soil  on  which  it  is  to  be  used. 
Each  brand  of  fertilizer  is  usually  composed  of  several  of 
the  constituents  that  have  been  described.  If  those  sub- 
stances are  used  that  are  difficultly  soluble,  the  ferti- 
lizer is  not  so  valuable  as  if  composed  of  easily  soluble 
substances.  The  solubility,  as  well  as  the  percentage 
of  each  ingredient  of  the  fertilizer,  should  be  known  by 
the  purchaser. 

A  fertilizer  is  known  in  the  market  as  a  high-grade  or 
a  low-grade  product,  depending  upon  the  percentage  of 
fertilizing  constituents  that  it  contains.  Low-grade 
fertilizers  are  cheaper  than  high-grade  merely  because 
they  contain  less  plant-food,  although  the  price  per 
pound  of  plant-food  may  be  no  less, — and,  in  fact,  is 
usually  more.  The  low-grade  product  is  encumbered 
with  a  large  amount  of  inert  material,  that  adds  to  the 
cost  of  transportation  and  handling,  without  adding 
to  the  value  of  the  fertilizer.  For  these  reasons,  the  high- 
grade  material  is  almost  always  the  cheaper  fertilizer. 


344          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

A  ton  of  low-grade  fertilizer  may  contain  500  or  600 
pounds  more  inert  material  than  a  high-grade  fertilizer, 
upon  which  freight  must  be  paid,  and  which  must  be 
hauled  from  the  station  and  spread  upon  the  field. 

211.  Fertilizer  inspection. — Some  thirty  states  have 
enacted  legislation  providing  for  the  inspection  and  con- 
trol of  the  sale  of  commercial  fertilizers.    Each  package 
of  fertilizer  must  bear  a  certificate  stating  the  percentage 
of  nitrogen,  phosphoric  acid  and  potash,  and  more  or  less 
information  in  regard  to  the  forms  in  which  these  are 
held  and  their  rates  of  solubility.   This  must  be  guaran- 
teed to  be  correct  by  the  manufacturer. 

The  guarantee  does  not  always  state  the  percentage 
of  nitrogen  (N),  phosphoric  acid  (P205),  and  potash 
(K20),but  often  uses  other  terms  that  imply  the  presence 
of  these  substances,  but  so  combined  that  the  percentage 
of  the  carrier  is  larger, — as,  for  instance,  ammonia,  bone 
phosphate  and  sulfate  of  potash.  To  convert  one  term 
into  another,  factors  have  been  devised  which  greatly 
simplify  the  process. 

Per  cent  ammonia  X  .8235  =  per  cent  nitrogen  (N.) 
Per  cent  nitrate  of  soda  X  .1647  =  per  cent  nitrogen  (N). 
Per  cent  bone  phosphate  X  .458  =  per  cent  phosphoric  acid 

(PA). 

Per  cent  muriate  of  potash  X  .632  =  per  cent  potash  (K,O). 
Per  cent  sulfate  of  potash  X  .54  =  per  cent  potash  (KaO). 

212.  Trade  values  of  fertilizers. — It  has  been  custom- 
ary for  the  authorities  charged  with  fertilizer  inspection 
in  the  states  concerned  to  adopt  each  year  a  schedule  of 
trade  values  for  nitrogen,  phosphoric  acid   and   potash, 
in  each  of  the  various  forms  in  which  they  appear  in 


TRADE  VALUE  OF  FERTILIZERS        345 

fertilizers.  These  values  are  based  on  the  cost  of  the 
unmixed  constituents,  if  purchased  in  wholesale  lots 
from  the  manufacturer,  and  are  secured  by  averaging 
the  wholesale  prices  per  ton  of  all  the  various  fertilizer 
supplies  for  the  six  months  preceding  March  1,  to  which 
is  added  about  20  per  cent  of  the  price,  to  cover  cost  of 
handling.  The  trade  values  for  1907  were  as  follows: 

Value  per  pound 

Cents 
Nitrogen,  in  nitrates 18.5 

Nitrogen,  in  ammonium  salts 17.5 

Organic  nitrogen,  in  dried  and  finely  ground  fish  meat 

and  blood,  and  in  mixed  fertilizers 20.5 

Organic  nitrogen,  in  finely  ground  bone  and  tankage.  .20.5 
Organic  nitrogen,  in  coarsely  ground  bone  and  tankage.  15.0 

Phosphoric  acid,  soluble  in  water    5.0 

Phosphoric  acid,  soluble  in  ammonium  citrate 4.5 

Phosphoric  acid,  insoluble,  in  fine  bone  and  tankage.  .    4.0 
Phosphoric  acid,  insoluble,  in  coarse  bone  and  tankage  .   3.0 

Phosphoric  acid,  insoluble,  in  mixed  fertilizers 2.0 

Phosphoric  acid,  insoluble,  in  finely  ground  fish,  cotton- 
seed meal,  castor  pomace  and  wood-ashes    4.0 

Potash,  as  muriate 4.5 

Potash,  as  sulfate,  and  in  forms  free  from  muriates.  .   5.0 

213.  Computation  of  the  commercial  value  of  a  ferti- 
lizer.— The  percentage  of  each  fertilizing  constituent  of  a 
fertilizer,  and  its  form  or  rate  of  solubility  being  known,  it 
is  possible  to  calculate  its  commercial  value.  Suppose  a 
fertilizer  costing  $4S  per  ton  contains  the  following: 

Per  rent 

Nitrogen  in  sodium  nitrate 4 

Nitrogen  in  fine  bone 3 

Phosphoric    acid,   available,    in     rock    superphosphate 
(corresponds  to  soluble  in  ammonium  citrate).  ...   6 

Phosphoric  acid,  insoluble,  in  fine  bone 22 

Potash,  water  soluble,  in  muriate  of  potash 10 


346         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

The  number  of  pounds  of  each  constituent  per  ton 
of  fertilizer  is  then  found  thus: 

Nitrogen  as  nitrate 4x20=   80  pounds  per  ton 

Nitrogen  in  fine  bone 3x20=  60  pounds  per  ton 

Phosphoric  acid,  available. .  .  .  6X20  =  120  pounds  per  ton 
Phosphoric  acid,  insoluble  .  .  .22X20  =  440  pounds  per  ton 
Potash,  muriate 10X20  =  200  pounds  per  ton 

The  trade  values,  as  published  by  the  fertilizer  in- 
spection authorities,  are  then  applied  to  the  several 
constituents. 

Nitrogen,  as  nitrates 80X .185  =  $14  80 

Nitrogen  in  fine  bone 60  X  .205  =    12  30 

Phosphoric  acid,  available 120  X  .045=     5  40 

Phosphoric  acid,  insoluble 43  X  .040=     1  60 

Potash,  muriate 200  X  .045  =     9  00 

$43  10 

The  computed  value  may  then  be  compared  with  the 
market  price.  It  must  be  remembered  that  this  is  the 
commercial  value,  and  not  necessarily  the  agricultural 
value,  which  is  determined  by  the  profits  from  its  use, 
and  will  depend  upon  many  other  factors.  For  instance, 
a  soil  markedly  deficient  in  nitrogen  will  not  respond  to  a 
phosphate  fertilizer  alone  to  an  extent  which  would 
justify  its  use. 

214.  Mixing  fertilizers  on  the  farm. — It  has  been 
shown  by  several  of  the  Experiment  Stations  that  the 
raw  materials  may  be  purchased  from  the  manufacturers 
and  mixed  on  the  farm  at  a  considerably  lower  cost  than 
they  can  be  bought  in  fertilizer  mixtures,  and  that  the 
results  obtained  from  them  are  fully  as  satisfactory. 


METHODS   OF   APPLYING   FERTILIZERS  347 

Other  advantages  from  home-mixing  are,  that  it  per- 
mits the  farmer  to  use  exactly  the  proportions  of  the 
several  constituents  that  he  desires,  and  that  it  makes 
unnecessary  the  handling  of  a  large  amount  of  inert 
material  frequently  contained  in  mixed  fertilizers. 
It  is  thus  possible  for  him  to  ascertain  by  fields  test 
the  best  proportions  of  the  various  fertilizer  constituents 
to  use  upon  his  own  land  for  each  of  the  crops  he  is  grow- 
ing, which  knowledge  makes  it  possible  to  decrease 
greatly  the  expenditure  for  fertilizers. 

215.  Methods  of  applying  fertilizers. — The  distribu- 
tion of  the  fertilizer  by  means  of  machinery  is  much  more 
satisfactory  than  is  broadcasting  by  hand,  as  the  former 
method  gives  a  much  more  uniform  distribution.  Cereals 
and  other  crops  planted  with  a  drill  or  planter  are  now 
usually  provided  with  an  attachment  for  dropping  the 
fertilizer  at  the  same  time  that  the  seed  is  sown,  the  ferti- 
lizer being  by  this  method  placed  under  the  surface  of  the 
soil.  Broadcasting  machines  are  also  used,  which  leave 
the  fertilizer  uniformly  distributed  on  the  surface  of  the 
ground,  thus  permitting  it  to  be  applied  anil  harrowed-in 
sufficiently,  before  the  seed  is  planted,  to  prevent  injury 
to  the  seed  by  the  chemical  activity  of  the  fertilizing 
material. 

Corn  planters  with  fertilizer  attachments  deposit  the 
fertiliser  beneath  the  seed,  so  as  not  to  bring  the  two  in 
contact,  drain  drills  do  not  do  tlu's.  and.  where  the 
amount  of  fertilizer  used  exceeds  UOO  or  400  pounds  per 
acre,  it  is  better  to  apply  it  before  seeding.  Grass  seed 
and  other  small  seeds  should  be  planted  only  after  the 
fertilizer  has  been  mixed  with  the  soil  for  several  davs. 


348         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

For  crops  to  which  large  quantities  of  fertilizers  are  to 
be  added,  it  is  desirable  to  drop  only  a  portion  of  the 
fertilizer  with  the  seed,  the  remainder  to  be  broadcasted 
by  machinery  and  harrowed  in  earlier,  and,  as  is  fre- 
quently better  for  crops  requiring  very  liberal  fertiliza- 
tion, a  later  application  may  be  made. 

216.  Soil  amendments. — Certain  substances  are  some- 
times added  to  soils  for  the  purpose  of  increasing  pro- 
ductiveness   through    their    influence    on    the    physical 
structure  of  the  soil,   and  thereby  upon  the  chemical 
and   bacteriological    properties.     These   substances    are 
called  soil  amendments.    It  is  true  that  they  may  add 
essential  plant  ingredients  to  the  soil,  but  that  function 
is  of  minor  importance. 

217.  Salts  of  calcium. — Calcium,   although  essential 
to  plant  growth,  need  seldom  be  added  to  the  soil  to 
supply  the  plant  directly;  but,  on  account  of  its  effect 
upon  the  soil  properties,  its  use  is  beneficial  to  a  great 
number  of  soils. 

218.  Effect  on  tilth  and  bacterial  action. — On  clay 
soils,  the  effect  of  lime  is  to  bring  the  fine  particles  into 
aggregates  which  are  loosely  cemented  by  the  calcium 
carbonate.    The  effect  of  this  structure  upon  tilth  has 
already  been  explained.   (Seepage    117.)  On  sandy  soils, 
the  carbonate  of  calcium  serves  to  bind  some  of  the  par- 
ticles together,  making  the  structure  somewhat  firmer, 
and  increases  the  water-holding  power.     It  should  be 
used  only  in  small  amounts  on  sandy  soils. 

There  is  a  tendency  for  most  cultivated  soils  to  be- 
come acid,  owing  to  the  formation  of  organic  acids  in 
decomposition  and  to  the  greater  removal  of  mineral 


SOIL   AMENDMENTS  349 

bases  than  acids  by  plants,  but  particularly  because 
of  the  loss  of  lime  and  the  alkali  salts  in  the  drainage 
water.  Acidity  may  reach  a  point  where  it  becomes 
directly  injurious  to  certain  plants,  but  it  becomes 
indirectly  injurious  before  that  point  is  reached.  One 
way  in  which  this  occurs  is  by  curtailing  the  action  of 
certain  bacteria  in  their  processes  of  rendering  plant-food 
available.  A  slightly  alkaline  reaction  and  an  easily 
available  base  to  combine  with  the  organic  acids  affords 
the  most  favorable  condition  for  the  decomposition 
processes  due  to  bacterial  action,  and  hence  the  best 
results  cannot  be  obtained  where  carbonate  of  lime  is  not 
present.  Its  action  in  improving  tilth  also  facilitates 
desirable  forms  of  bacteriological  activity  by  increas- 
ing the  permeability  of  the  soil  for  air. 

219.  Liberation  of  plant-food  materials. — It  has  been 
stated  (page  297)  that  the  alkalies  and  alkaline  earths  are 
more  or  less  interchangeable  in  certain  compounds  in 
the  soil.  The  addition  of  lime  may  in  this  way  liberate 
potassium,  when  otherwise  it  would  be  difficult  for 
crops  to  obtain  a  sufficient  supply  from  a  particular 
soil.  Magnesium,  although  rarely  deficient,  may  also 
be  made  available  in  this  way.  The  use  of  calcium  salts 
may  also  render  phosphorus  more  useful,  probably  by 
supplying  a  base  more  soluble  than  iron  or  alumina 
with  which  in  soils  deficient  in  calcium  the  phosphorus 
might  otherwise  be  combined. 

Boussingault,  as  quoted  by  Storer,  found  that  the 
addition  of  lime  to  a  clover  crop  increased  greatly  the 
calcium,  potassium  and  phosphorus  contained  in  the 
crop. 


350          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 
TABLE  IL 


Kilos  per  hectare 

Lime 

Potash 

Phos- 
phoric 
acid 

Crop  not  limed  (first  year)    

32.2 
79.4 
32.2 
102.8 

26.7 
95.6 
28.6 
97.2 

11.0 

24.2 
7.0 
22.9 

Crop  limed  (first  year)  

Crop  not  limed  (second  year)  

Crop  limed  (second  year)  .  .    . 

Calcium  salts  may  also  increase  greatly  the  rate  at 
which  nitrogen  becomes  available  by  its  effect  upon 
bacterial  action,  as  before  explained. 

220.  Effect  on  toxic  substances  and  plant  diseases.— 
Free  acids  are  toxic  to  most  agricultural  plants.  Some 
plants  are  much  more  sensitive  than  others.  Clover  and 
alfalfa,  for  instance,  should  have  a  slightly  alkaline 
medium  for  their  best  growth,  and  any  acid  is  very 
injurious.  Calcium  salts  in  neutralizing  acidity  remove 
this  toxic  condition. 

Certain  toxic  substances  of  an  organic  nature  are 
also  said  to  be  rendered  innocuous  by  the  presence  of 
calcium  carbonate.  Magnesium  salts,  when  present  in 
excess,  may  exert  a  toxic  action  upon  plants.  The 
relative  proportion  of  calcium  and  magnesium,  accord- 
ing to  Loew,  determines  whether  or  not  magnesium  is 
toxic.  The  exact  limits  of  the  ratio  of  magnesium  to 
calcium  beyond  which  the  former  is  toxic  depends  upon 
the  combinations  and  solubilities  of  the  two,  and  also 
upon  the  crop  grown.  An  actually  greater  amount  of 
magnesia,  as  shown  by  a  strong  hydrochloric  acid  diges- 


L/.VE    AS   A    SOIL   AMEXDMEXT  351 

tion  analysis,  is  not  present  in  very  fertile  soils  of  any 
region,  according  to  Loew.  If  injury  from  magnesium 
is  suspected,  the  obvious  means  of  correction  is  to 
increase  the  proportion  of  calcium  by  its  addition  in 
some  form. 

The  use  of  limestone,  ground  or  burned,  that  contains 
a  large 'percentage  of  magnesium  may  be  injurious  to 
some  soils,  as  may  also  those  Stassfurt  salts  containing 
magnesium. 

The  presence  of  soluble  calcium,  with  its  effects 
upon  the  soil,  retards  the  development  of  certain  plant 
diseases,  like  the  "finger  and  toe"  disease  of  the  cruci- 
ferae.  On  the  other  hand,  it  may  promote  some  diseases, 
as,  for  instance,  the  potato  "scab." 

221.  Forms  of  calcium. — Calcium  is  used  on  the 
soil  in  the  form  of  calcium  oxide,  or  quicklime  (CaO), 
water-slaked  lime  (Ca(OH)2),  air-slaked  lime  (CaCO,), 
ground  limestone  (also  a  carbonate),  and  calcium  sulfate, 
or  gypsum  (CaSO,,  2H,O).  The  application  of  any  of 
these  is  usually  called  Hming  the  soil,  although  gypsum 
does  not  serve  exactly  the  same  purpose  as  do  the  other 
forms.  Owing  to  differences  in  the  molecular  weights 
of  these  compounds  of  calcium,  it  requires  more  of  some 
forms  than  of  others  to  furnish  the  same  amount  of 
calcium.  Approximately  equivalent  quantities  of  some 
of  the  common  forms  when  fairly  pure  are: 

Quicklime ">l>  pounds 

Water-slaked  lime 74  pounds 

Air-slaked  lime,  marl  and  ground  limestone.  .      100  pounds 

Caustic  lime,  or  the  hydrate,  when  added  to  the  soil, 
eventuallv  assume  some  of  the  more  insoluble  forms  of 


352         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

combination  or  remain  as  the  carbonate,  never  being 
present  as  the  oxide.  It  is  always  desirable  to  have 
present  in  the  soil  at  least  a  small  amount  of  calcium 
carbonate. 

222.  Caustic  lime. — Quicklime  and  water-slaked  lime 
have  a  markedly  alkaline  reaction,  and  hence  neutralize 
quickly  any  acidity  that  may  exist  in  the  soil.   They  act 
also  quickly  in  liberating  plant-food,  particularly  nitro- 
gen. Some  soils  respond  more  rapidly  to  quick-  or  water- 
slaked  lime  than  to  carbonate  of  lime,  especially  when 
the  carbonate  is  in  the  form  of  marl  or  ground  limestone, 
in  which  cases  it  is  never  in  such  a  finely  pulverized  con- 
dition.   The  use  of  the  caustic  forms  of  lime  has  been 
said  to  result  in  the  loss  of  nitrogen  by  the  decomposition 
of  organic  compounds. 

Upon  clays,  the  granulating  effect  of  caustic  lime  is 
more  marked  than  that  of  the  carbonate,  and  for  this 
reason  the  former  has  a  distinct  advantage  for  use  on 
heavy  clay.  An  occasional  moderate  dressing  is,  for  the 
same  reason,  better  than  a  heavy  dressing  given  less 
frequently. 

223.  Carbonate   of   lime. — Air-slaked   lime   has   the 
advantage  of  being  in  a  finely  divided  condition,  and 
does   not   produce   the   injurious   action   upon   organic 
matter  attributed  to  caustic  lime.    Its  effect  upon  the 
granulation  of  clay  soils  is  probably  less  pronounced 
than  that  of  caustic  lime. 

Marl  differs  from  air-slaked  lime  principally  in  its 
property  of  being  in  a  less  finely  pulverized  condition. 
It  acts  less  quickly  than  does  caustic  lime.  Owing  to 
the  fact  that  marl  deposits  differ  greatly  in  the  compo- 


FORMS  OF   LIME  AND   CROP    VALUE  353 

sition  of  their  products,  it  is  well  to  know  the  quality 
of  the  material  before  purchasing  it.  The  carbonate  of 
lime  in  marl  may  vary  from  5  or  10  to  90  or  95  per  cent 
in  different  samples. 

Ground  limestone  has  been  used  as  a  substitute  for 
marl.  It  is  very  important  that  it  be  finely  ground,  as 
upon  the  comminution  of  the  material  much  of  its  effi- 
ciency depends.  As  there  was  some  question  as  to  the 
value  of  ground  limestone,  experiments  in  which  it  was 
compared  with  caustic  lime  have  been  conducted  at 
some  of  the  experiment  stations.  These  have,  in  the 
main,  given  results  very  favorable  to  finely  ground  lime- 
stone. 

Frear  reports  tests  in  which  plats  treated  with  slaked 
lime,  at  the  rate  of  two  tons  per  acre  once  in  four  years, 
were  compared  with  plats  treated  with  ground  limestone, 
at  the  rate  of  two  tons  per  acre  every  two  years.  The 
records,  at  the  end  of  twenty  years,  show  that  in  every 
case  the  total  yields  were  greater  on  the  plats  receiving 
ground  limestone.  After  the  treatment  on  these  plats 
had  been  continued  for  sixteen  years,  a  determination  of 
nitrogen  showed  the  upper  nine  inches  of  soil  on  the 
limestone-treated  plats  to  contain  2,979  pounds  of  nitro- 
gen per  acre,  and  the  slaked-lime  plats  to  contain  2,(>04 
pounds.  It  may  be  inferred  from  these  figures  that  the 
slaked  lime  caused  a  greater  destruction  of  organic 
matter  than  did  the  limestone. 

Patterson  also  conducted  experiments  for  eleven 
years  with  caustic  lime  produced  by  burning  both  stone 
and  shells,  and  the  carbonate  of  lime  in  ground  shells 
and  shell  marl.  The  average  crops  of  mai/.e,  wheat 

w 


354         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

and    hay   were   all    larger    on   the    carbonate-of-lime 
treated  plats. 

224.  Sulfate  of  lime. — Gypsum,  in  which  form  calcium 
sulfate  is  usually  applied  to  soils,  is  effective  in  liberating 
potash,  and  possibly  other  substances,  from  the  more 
difficultly  soluble  combinations.   Its  action  in  improving 
tilth  is  less  marked  than  that  of  caustic  lime,  or  of  the 
carbonate.     Whether  it   eventually   contributes  to  the 
presence  of   carbonate  of  lime  is   a   matter  regarding 
which  there  is  still  a  difference  of  opinion.    It  has  the 
disadvantage  of  introducing  into  the  soil  an  acid  radical, 
which  is  removed  by  plants  only  in  small  amounts,  and 
which  tends  to  produce  an  acid  condition  of  the  soil. 
On  the  whole,  gypsum  is  not  an  adequate  substitute  for, 
nor  so  desirable  a  form  of,  calcium  as  the  oxide,  hydroxide 
or  carbonate. 

225.  Common  salt. — Sodium  chloride  has  a  marked 
effect  upon  some  soils,  but  wherein  its  effectiveness  lies 
is  not  well  understood.    The  addition  of  sodium  and  of 
chlorine  as  plant  constituents  is  clearly  not  the  reason, 
as  these  substances  are  always  present  in  soils  in  avail- 
able form  far  in  excess  of  their  requirements. 

The  effect  of  sodium  chloride  upon  clay-bearing  soils 
is  to  liberate  certain  plant  nutrients,  among  which  are 
calcium,  magnesium,  potassium,  calcium  and  phos- 
phorus. This  action,  although  limited  in  amount,  is, 
in  some  cases  at  least,  partly  responsible  for  the  bene- 
ficial action  of  common  salt. 

The  structure  of  the  soil  is  improved  by  the  applica- 
tion of  sodium  chloride,  just  as  it  is  by  lime, — although 
usually  not  to  the  same  extent. 


OTHER   SOIL   AMENDMENTS  355 

Another  effect  of  salt  is  to  conserve  and  distribute 
soil  moisture.  Its  conserving  action  is  probably  due  to 
an  increase  in  the  density  of  the  soil-water  solution  re- 
tarding transpiration.  The  film  movement  of  water  is 
likewise  increased  by  the  presence  of  salt  in  the  solution, 
and  in  this  way  the  upward  movement  of  bottom  water 
is  facilitated,  and  the  supply  within  reach  of  the  roots 
maintained  in  time  of  drought. 

It  is  not  all  soils,  however,  that  are  benefited  by  salt, 
its  usefulness  not  being  of  such  wide  application  as  that 
of  lime.  Certain  crops,  as  previously  mentioned  (page 
340),  are  injured  by  the  presence  of  chlorine. 

226.  Muck. — The  effect  of  muck  is  to  change  the 
structure  of  soils;  making  heavy  clay  soils  lighter  and 
more  porous,  and  binding  together  the  particles  of  a 
sandy  one.  Both  classes  of  soils,  but  particularly  the 
sandy  type,  have  a  greater  water-holding  capacity  after 
treatment  with  muck,  owing  to  its  great  absorptive 
power,  amounting  to  70  per  cent  or  more  of  its  own 
weight.  (See  page  153.)  It  is  to  its  content  of  organic 
matter  that  the  physical  effects  of  muck  are  due. 

Muck  contains  1.0  to  2.0  per  cent  of  organic  nitrogen, 
calculated  to  dry  matter  which  does  not  readily  undergo 
ammonification.  The  addition  of  farm  manure  which 
ferments  readily,  and  of  lime,  serves  to  hasten  ammoni- 
fication. Its  use  as  an  absorbent  in  the  stable  fits  it  well 
for  use  on  the  land. 

Very  large  applications  of  muck  are  necessary  when 
it  is  used  to  improve  the  structure  of  the  soil.  From 
ten  to  forty  or  fifty  tons  per  acre  are  frequently 
applied. 


356          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

227.  Factors  affecting  the  efficiency  of  fertilizers.— 

The  potentially  available  nutrients  in  a  soil,  whether 
natural  or  added  in  manures  or  fertilizers,  are  only  in 
part  utilized  by  plants,  and  the  extent  of  their  utilization 
depends  upon  the  operation  of  certain  limiting  factors. 
This  is  a  very  important  consideration  in  the  manuring  of 
land,  for  under  conditions  as  they  frequently  exist  the 
use  of  fertilizers  is  wasteful  and  extravagant. 

The  factors  within  the  control  of  man  that  effect  the 
availability  of  fertilizing  material  are  the  following: 
(1)  Soil  moisture  content.  (2)  Soil  acidity.  (3)  Organic 
matter  in  the  soil.  (4)  Structure  or  tilth  of  the  soil. 

An  undesirable  condition  of  any  one  or  more  of  these 
factors  is  a  very  common  and  apparent  occurrence,  and 
yet  fertilizers  are  expected  to  produce  profitable  returns, 
in  spite  of  these  adverse  conditions.  It  must  be  remem- 
bered that  fertilizers  are  primarily  only  nutrient  materials, 
and  that  the  supply  of  nutrients  is  only  one  of  the  con- 
ditions that  influence  plant  growth.  Furthermore,  an 
economical  use  of  fertilizers  requires  that  they  merely 
supplement  the  natural  supply  in  the  soil,  and  that  the 
latter  should  furnish  the  larger  part  of  the  soil  material 
used  by  the  crop.  Finally,  most  fertilizers  are  ren- 
dered more  or  less  difficultly  soluble,  or  in  some  cases 
practically  insoluble  in  pure  water,  by  the  absorptive 
properties  of  the  soil,  and  the  release  of  these  sub- 
stances for  plant  use  depends  to  a  great  extent  upon  the 
factors  mentioned  above. 

For  instance,  when  a  potassium  fertilizer,  as  potas- 
sium sulfate  or  chloride,  is  placed  in  the  soil,  a  consider- 
able portion  of  the  potassium  is  (page  297)  fixed  by  ab- 


EFFICIENCY   OF   FERTILIZERS  357 

sorption  as  one  of  the  bases  in  a  poly-silicate,  and  thus 
held  in  a  condition  very  sparingly  soluble  in  pure  water. 
Other  reactions  take  place,  and  a  portion  of  the  potas- 
sium in  some  form  is  doubtless  mechanically  held  by  the 
soil  particles.  While  this  added  potassium  is  more 
readily  obtained  by  plants  than  that  contained  naturally 
in  many  soils,  it  must  become  available  largely  by  the 
processes  by  which  the  natural  supply  is  rendered  soluble. 
Ammonium  sulfate  undergoes  a  somewhat  similar  pro- 
cess, while  the  nitrate  of  soda  remains  in  a  soluble  form. 

It  is  evident,  therefore,  that  the  conditions  which 
contribute  to  the  natural  fertility  of  the  soil  also  apply 
to  that  added  as  fertilizers,  with  the  possible  exception 
of  the  nitrate. 

Phosphate  fertilizers  may  be  rendered  practically 
insoluble  in  pure  water,  when  added  to  the  soil,  and  in 
the  presence  of  a  large  amount  of  iron  and  aluminum  it 
forms  more  or  less  ferric  and  aluminum  phosphate, 
which  becomes  soluble  very  slowly,  even  under  the 
action  of  soil-water  and  plant-roots.  When  converted 
into  tricalcium  phosphate,  the  phosphorus  becomes 
soluble  more  readily;  but,  in  any  case,  its  rate  of  solu- 
bility depends  upon  those  conditions  which  are  most 
favorable  to  the  solubility  of  the  natural  soil  phosphates. 

It  is  generally  recognized  that  a  sandy  soil  responds 
more  promptly  to  the  application  of  fertilizers  than  does 
a  clay  soil.  There  may  be  two  reasons  for  this:  (1) 
Absorption  may  not  be  so  complete  both  on  account  of 
the  particles  being  larger,  and  because  in  many  sandy 
soils  the  particles  are  largely  composed  of  quartz,  which 
does  not  have  the  property  of  forming  combinations 


358          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

with  bases  as  does  clay.  (2)  Drainage  and  aeration  are 
likely  to  be  better,  as  are  all  those  conditions  that  con- 
duce to  solubility  of  plant-food.  For  these  reasons,  a 
sandy  soil  generally  gives  larger  returns  the  first  year 
from  the  application  of  manures,  but  shows  less  effect 
in  subsequent  years  unless  the  treatment  is  repeated. 
Clay  soils  are,  for  these  reasons,  more  likely  to  involve 
a  wasteful  use  of  fertilizers  than  are  sandy  soils,  except 
in  respect  to  loss  of  nitrogen  in  drainage,  in  which  the 
sandy  soil  is  more  likely  to  be  at  fault,  especially  if  there 
is  no  crop  on  the  land. 

228.  Soil-moisture  content. — Soils  in  a  humid  region 
commonly  suffer  from  an  excess  of  water  in  the  spring, 
and  a  deficiency  in  the  summer.  Cereals  and  many  other 
crops  require  the  largest  quantity  of  water  at  the  time 
of  heading  and  blossoming,  and  the  largest  production 
of  crop  can  be  secured  only  where  the  supply  is  adequate 
at  that  time.  It  is  safe  to  say  that  in  the  great  majority 
of  cases  crops  raised,  even  in  the  humid  region,  suffer 
at  some  time  from  a  deficient  water-supply.  On  the  other 
hand,  it  is  well  known  that  crops,  almost  without  excep- 
tion, suffer  either  by  lateness  of  planting,  or  by  delayed 
early  growth  from  an  excess  of  moisture  in  the  spring. 

A  control  of  the  soil-moisture  supply  should,  there- 
fore, remove  the  excess  of  moisture  in  a  time  of  large 
rainfall,  and  conserve  it  in  time  of  drought. 

There  are  three  means  that  may  be  employed  to 
bring  this  about:  (1)  Drains,  especially  by  means  of  tile. 

(2)  Use   of   green    manures   or   other   organic    matter. 

(3)  Good  tillage.    (See  page  190.) 

Viewed  purely  from  the  standpoint  of  soil  fertility, 


EFFICIENCY    OF   FERTILIZERS  359 

tile  drainage  does  much  to  increase  crop  production, 
and  to  effect  economy  in  the  use  of  fertilizers.  The  rela- 
tion of  soil  drainage  to  soil  fertility  may  be  summarized 
as  follows.  (See,  also,  page  239.) 

(1)  Aeration    provided    by    the    removal    of    water 
greatly  facilitates   nitrification.   This  relieves  the  con- 
stant necessity  for  the  use  of  soluble  nitrogen  fertilizers, 
and  makes  it  possible  to  rely  largely  upon  the  use  of 
leguminous   crops   for   nitrogen   fertilization.     Aeration 
also  renders  the  other  fertilizing  constituents  of  the  soil 
more  easily  soluble. 

(2)  By  quickly  removing  the  excess  moisture  in  the 
early  spring,  and  thus  increasing  the  length  of  the  grow- 
ing period,  plants  secure  more  nutriment,  there  is  a  cor- 
responding increase  in  the  length  of  time  in  which  nitri- 
fication  can   take   place,   also   in   other  action   brought 
about   by   aeration.     Available   nitrogen   thus   produced 
at  an  early  period  in  the  crop  growth  is  more  effective 
than  a  later  supply  would  be. 

(3)  By  removing  an  excess  of  water  from  the  soil,  a 
larger  proportion  of  the  available  fertility,  both  natural 
and  that  added  in  manures,  is  absorbed  by  the  crop. 
This  is  because  the  solution  is  less  dilute,   and    conse- 
quently   a    larger    amount    of    mineral    nutrients    pass 
through  the  plant  by  transpiration. 

229.  Soil  acidity. — An  acid  condition  of  the  soil 
renders  ineffective  a  large  proportion  of  the  fertilizing 
material  that  might  otherwise  be  available.  A  good 
illustration  of  this  is  the  comparison  of  the  crops  grown 
on  acid  soil  when  treated  with  lime  with  a  similar  soil 
not  so  treated.  The  size  of  the  crop  on  contiguous  plats 


360         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

has  been  increased  several  hundred  per  cent  by  the  use 
of  lime  at  a  number  of  the  Experiment  Stations.  The 
amount  of  the  acidity  determines  the  injury  it  occasions. 
There  is  always  a  great  waste  of  fertilizers  when  they  are 
added  to  an  acid  soil.  The  acidity  should  be  corrected 
by  the  application  of  lime,  in  order  that  manuring  shall 
be  most  effective. 

There  are  several  ways  in  which  an  acid  condition 
of  the  soil  operates  to  render  ineffective  the  natural 
and  applied  fertility. 

(1)  Bacteria  which  are  concerned  in  the  processes  of 
rendering   plant-food   available   do   not   usually   thrive 
in  an  acid  media,  preferring  a  neutral  or  slightly  alka- 
line condition.     Acidity  for  this  reason  checks  nitrifi- 
cation, as  well  as  the  bacteriological  processes  by  which 
phosphorus  is  rendered  soluble. 

(2)  Bacteria  concerned  in  the  acquisition  of  atmos- 
pheric nitrogen  in  symbiosis  with  legumes  are  greatly 
injured  by  an  acid  condition  of  the  soil.    Nitrogen  con- 
servation, one  of  the  most  important  features  of  the  use 
of  legumes  for  green  manuring,  cannot  be  effectively 
carried  out  on  an  acid  soil. 

(3)  The  liberation  of  potassium  from  zeolitic  combi- 
nations is  best  effected  only  where  there  is  a  basicity 
that  will  permit  the  replacement  of  one  base  by  another. 
The  presence  of  at  least  a  small  amount  of  calcium  car- 
bonate in  the  soil  is  essential  for  this,  as  it  is  for  many 
other  desirable  processes,  and  an  acid  condition  of  the 
soil  means  that  no  basicity  exists. 

(4)  Lime,    when    present     in    large    amount,    reacts 
with  the  very  insoluble  phosphates  of  iron  and  alumina, 


EFFICIENCY   OF   FERTILIZERS  361 

and  by  producing  phosphate  of  lime,  renders  the  phos- 
phoric acid  more  available  for  the  plant. 

230.  Organic  matter. — The  ways  in  which  organic 
matter  contributes  to  economy  in  the  use  of  fertilizers 
are:  (1)  By  improving  the  soil  structure.  (2)  By  con- 
serving moisture.  (3)  By  producing  through  decompo- 
sition carbon  dioxide  which,  dissolved  in  water,  is  a 
weak  but  continuously  acting  solvent  of  the  mineral 
fertilizers;  also  by  forming  organic  acids  that  act  in  a 
similar  way.  (4)  It  furnishes  a  source  of  food  and  energy 
for  bacteria,  which  aid  in  rendering  soluble  the  absorbed 
fertilizing  constituents. 

It  is  particularly  in  rendering  available  to  plants  the 
more  difficultly  soluble  phosphate  fertilizers  that  organic 
matter  directly  aids  in  making  fertilizers  more  effective. 

Farm  manure  is  undoubtedly  the  best  all-round  ferti- 
lizer to  be  had.  In  addition  to  adding  organic  matter  and 
certain  mineral  plant-food  materials,  it  introduces  into 
the  soil,  and  furnishes  a  favorable  medium  for  the  growth 
of  large  numbers  of  bacteria  that  are  of  great  value  in 
rendering  available  the  plant  nutrients  contained  in  soils. 

The  use  of  raw  or  untreated  phosphates  to  replace 
superphosphates  in  soil  manuring  has  received  much 
attention  in  Germany  and  to  some  extent  in  this  country 
in  recent  years.  Raw  phosphates,  being  much  more 
difficultly  soluble  than  the  superphosphates,  do  not, 
under  most  conditions  of  the  soil,  give  as  marked 
returns.  On  the  other  hand,  the  raw  phosphate  has  the 
advantage  of  being  very  much  cheaper,  and  of  not  con- 
taining sulfuric  acid.  The  extent  to  which  raw  phosphates 
will  become  available  in  the  soil  depends  largely  on  the 


362 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


extent  of  decomposition  of  organic  matter.  A  soil  poor 
in  humus,  and  which  has  not  been  treated  with  farm 
manure  or  green  manure,  is  not  likely  to  respond  very 
strongly  to  an  application  of  raw  phosphate.  The  fact 
that  superphosphate  is  available  under  these  conditions 
is  likely  to  lead  to  its  use  without  any  attempt  to  im- 
prove the  humus-content  of  the  soil,  and  thus  increase 
those  difficulties  that  arise  from  a  deficiency  of  organic 
matter.  It  is  this  condition  that  makes  it  necessary 
to  constantly  increase  the  dressings  of  fertilizer  in  order 
to  -maintain  productiveness. 

Experiments  by  Thorne  have  shown  that  the  use  of 
farm  manure  in  conjunction  with  raw  phosphates  serves 
to  increase  greatly  the  availability  of  the  latter.  In 
these  experiments  stall  manure  was  used  at  the  rate  of 
eight  tons  per  acre,  in  one  case  alone,  and  in  another 
in  connection  with  320  pounds  of  rock  phosphate. 
The  manures  were  applied  to  clover  sod,  and  plowed 
under  for  maize  in  a  rotation  of  corn,  wheat  and  clover. 
In  the  following  table,  average  yields  from  the  manured 
plats  and  from  the  unmanured  ones  are  given. 

TABLE  L 
EFFECT  OF  STALL  MANURE  ON  AVAILABILITY  OF  ROCK  PHOSPHATE 


Average 
yield  eleven 
crops 
maize 

Average 
yield 
ten  crops 
wheat 

Average 
yield 
seven  crops* 
hay 

Stall  manure,  8  tons 
Stall  manure,  8  ton 
rock  phosphate,  .' 
acre  

per  acre  

Bushels 
57.7 

64.0 
34.6 

Bushels 
20.3 

25.6 
10.4 

Tons 
1.6 

2.2 
1.0 

s  per  acre  and 
120  pounds  per 

No  manure  

'    EFFICIENCY   OF   FERTILIZERS  363 

It  will  be  seen  from  this  table  that  the  combination 
of  stall  manure  and  rock  phosphate  produced  larger 
crops  than  did  the  same  quantity  of  stall  manure  alone; 
from  which  it  may  be  fairly  concluded  that,  under  these 
conditions,  the  raw  phosphate  becomes  available  to  an 
extent  sufficient  to  make  its  use  practical.  Whether 
raw  phosphate  can  be  used  without  supplementing 
them  with  superphosphate  will  depend  upon  the  natural 
fertility  of  the  soil  and  the  amount  of  decomposing 
organic  matter  it  contains. 

231.  Structure  or  tilth  of  the  soil. — Tillage  aids  the 
plant  in  several   ways  to  obtain  nutrients  from   ferti- 
lizers  added   to   the   soil:   (1)   By   promoting   aeration. 
(2)   By  permitting  the  plant-roots  to  come  in  contact 
with  a  large  area  of  soil.    (3)  By  conserving  moisture 
in  time  of  drought. 

232.  Cumulative    need    for    fertilizers. — It    is    often 
remarked  that  on  fertilized  soils  there  is  a  gradually 
increasing    need    for    greater    quantities    of    fertilizers. 
This  is  doubtless  the  case  in  many  instances,  and  arises 
from  neglect  of  other  factors  affecting  soil  productive- 
ness.   As  we  have  seen,  certain  fertilizers  induce  a  loss 
of  lime  from  the  soil,  which,  if  allowed  to  continue,  -equires 
an  increased  amount  of  fertilizer  to  maintain  the  yield 
of  crops.  Organic  matter  is  allowed  to  decrease  and  this, 
as  well  as  loss  of  lime,  causes  the  soil  to  become  compact 
and  poorly  aerated,   and  so,  one  bad  condition   leading 
to  another,  crops  become  poorer  in  spite  of  increased 
applications  of  fertilizer. 

233.  Farm    manures. — The    original    components    of 
farm  manure  are  the  solid  excreta  from  the  animal,  the 


364 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


urine,  usually  from  the  same  animal  or  animals,  and  the 
litter  used  as  bedding  and  also  for  the  purpose  of  absorb- 
ing the  liquid  manure  and  to  render  the  whole  easier  to 
handle.  As  these  constituents  differ  greatly  in  their 
physical  and  chemical  properties,  the  proportions  in 
which  they  exist  affect  appreciably  the  properties  of  the 
manure. 


FIG.  106.     A  striking  example  of  waste  of  manure.   Leaching  and  fermentation 
will  remove  over  half  of  its  value  in  six  months. 

234.  Solid  excreta. — The  solid  excreta  furnishes  most 
of  the  body  of  the  manure,  and  as  it  is  already  in  a  stage 
of  partial  decomposition,  and  in  a  condition  both  physi- 
cally and  chemically  to  favor  the  further  processes  of 
decomposition,  it  is  largely  to  this  constituent  that  the 
fermentative  action  of  manure  is  due.  It  is  particularly 
valuable  for  the  effect  it  has  upon  the  physical  condition 
of  the  soil  and  the  encouragement  it  gives  to  decompo- 
ition  processes. 

Chemically,  it  is  not  so  valuable  as  the  liquid  excreta. 


FARM    MANURES 


305 


It  represents,  in  part,  the  food  materials  that  have 
passed  undigested  through  the  alimentary  canal,  and 
also  the  secretions  this  has  received  on  the  way,  and 
these  substances  are  not  all  held  in  a  soluble  form,  as 
are  those  in  the  urine. 

Stoeckhardt    states    the    composition    of    the    solid 
excreta  of  different  farm  animals  to  be  as  follows: 

TABLE  LI 


Water 

O'ompo.sition  of  dr 

¥  matter 

Nitro- 
gen 

Phos- 
phoric 

:ii-iil 

Alkalies 

Horses  (winter  foot!)    

Per  cent 
70 
84 
80 
58 

Per  cent 
2.  OS 
1.87 

:ux) 

1.78 

Per  cent 

1.45 
1  .50 
2.25 
1.42 

Per  cent 
1  .25 
0.02 
2.50 
0.71 

Cows  (winter  food)  

Swine  (winter  food)  

Sheep  (two  pounds  hay  pei  day). 

Calculated  to  1,000  pounds  of  solid  excrement,  these 
figures  show  the  following  number  of  pounds  of  each 
constituent. 

TABLE  LI  I 


Water 

Nitro- 
gen 

Phos- 
phoric 
acid 

Horse     

Pounds 
700 

Pounds 

5.0 

Pound" 

Cow        

840 

3.0 

2.5 

Swine        

800 

6.0 

4.5 

Sheep        

580 

7.5 

6.0 

Alkalies 


Pounds 

:u> 

10 
5.0 
3.0 


The  smaller  percentage  of  water  in  the  sheep  excre- 
ment makes  it,  pound  for  pound,  the  richest  of  any.   Next 


366 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


to  it  stands  hog  excrement,  and  cow  excrement  is  the 
poorest  in  fertilizing  materials. 

235.  Urine. — The  urine  represents  a  portion  of  the 
food  which  has  been  digested  by  the  animal  and  excreted 
as  a  waste  product  through  the  kidneys.  The  proportion 
of  the  nitrogen  and  mineral  matter  retained  by  the  tis- 
sues depends  upon  the  age  of  the  animal  and  upon  the 
nature  of  the  food.  An  animal  receiving  a  large  amount 
of  easily  digestible  nitrogenous  food  excretes  more  nitro- 
gen in  the  urine  than  a  poorly  fed  animal. 

The  composition  of  urine,  as  given  by  Stoeckhardt  is 
as  follows: 

TABLE  LIII 


Water 

Composition  of  dry  matter 

Nitro- 
gen 

Phos- 
phoric 
acid 

Alkalies 

Horse  (hay  and  oats)  

Per  cent 
89.0 
92.0 
97.5 
86.5 

Per  cent 
10.9 
10.0 
12.0 
10.4 

Per  cent 

trace 
trace 
5.0 
3.7 

Per  cent 
13.6 
17.5 
8.0 
14.9 

Cow  (hay  and  potatoes)  

Swine  (winter  food)  

Sheep  (two  pounds  hay  per  day)   . 

These  figures  show  the  following  number  of  pounds 
of  each  constituent  in  1,000  pounds  of  urine. 

TABLE  LIV 


Water 

Nitro- 
gen 

Phos- 
phoric 
acid 

Alkalies 

Horse   

Pounds 
890 

Pounds 

12 

Pounds 

Pounds 
15 

Cow  

920 

8 

14 

Swine    

975 

3 

1.25 

2 

Sheep    

865 

14 

0.50 

20 

COMPOSITION   OF   ANIMAL    MANURES  367 

The  liquid  excreta  of  the  sheep  contains  in  a  given 
quantity  more  fertilizing  material  than  that  of  any  of  the 
other  animals. 

Comparing  the  solid  and  liquid  excreta  of  these 
animals  as  a  whole,  it  will  be  seen  that,  in  general,  the 
urine  is  richest  in  nitrogen  and  alkalies,  while  the  solid 
^xcrement  is  richest  in  phosphoric  acid. 

The  amount  and  composition  of  the  urine  is  more 
constant  than  that  of  the  solid  excrement.  Both  are 
influenced  by  the  character  and  amount  of  feed,  but  the 
urine  much  less  so  than  the  solid  excrement.  Kxperi- 
ments  conducted  at  the  Rothamsted  Experiment  Sta- 
tion have  shown  that  from  57  to  79  per  cent  of  the  total 
nitrogen  of  the  food  is  excreted  in  the  urine,  and  from 
10  to  22  per  cent  in  the  solid  excrement. 

236.  Litter. — The  use  of  a  bulky  absorbent,  like 
straw,  sawdust  or  leaves,  is  almost  universal  where  live 
stock  are  kept  in  a  stable.  This  is  useful  in  providing 
a  soft  bed  for  the  animal,  in  absorbing  the  liquid  excre- 
ment, in  lightening  the  manure,  making  it  easier  to 
handle,  less  likely  to  undergo  undesirable  fermentation, 
and  more  effective  in  improving  the  physical  condition 
of  heavy  soils. 

Straw  is  the  absorbent  usually  used,  and  is,  all 
things  considered,  the  most  satisfactory.  It  decomposes 
readily  in  most  soils  and,  in  decomposing,  adds  to  the 
soil  considerable  fertilizing  material.  Of  the  different 
kinds  of  straw,  oat  straw  has  the  greatest  fertilizing 
value.  A  ton  of  oat  straw  contains  about  10  pounds 
nitrogen,  4  pounds  phosphoric  acid,  2(5  pounds  of  potash, 
and  9  pounds  of  lime.  As  this  is  more  nitrogen  and 


368         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

potash  than  is  contained  in  a  ton  of  average  manure, 
the  use  of  this  absorbent  increases  the  fertilizing  value 
of  the  manure.  It  is,  however,  undesirable  on  some  soils 
to  have  a  very  large  proportion  of  straw,  on  account 
of  its  effect  in  retarding  decomposition. 

Sawdust  and  shavings  are  sometimes  used,  but, 
while  they  are  good  absorbents,  they  decompose  very 
slowly  in  the  soil,  making  them  objectionable  on  light 
soils,  and  they  have  practically  no  plant-food  materials. 
Dry  leaves  absorb  well,  and  decompose  satisfactorily  in 
the  soil.  They  do  not  add  much  fertility. 

237.  Manures  produced  by  different  animals. — There 
is  a  great  difference  in  the  amount  and  value  of  manure 
produced  by  different  kinds  of  live  stock.   This  is  due  to 
a  number  of  causes,  among  which  are  the  size  of  the 
animal,  the  nature  of  its  food,  and  the  mechanical  con- 
dition in  which  the  digestive  processes  leave  the  solid 
excrement.    The  differences  affect  not  only  the  amount 
of  fertilizing  constituents  in  the  manures,  but,  what  is  of 
more  importance,  they  determine  the  nature  and  rapidity 
of  the  decomposition  processes,  and  hence  affect  the  loss 
of  manurial  substances  and  the  value  of  the  manure  as  a 
fermentive  agent  in  the  soil. 

238.  Horse  manure. — A  well-fed,  moderately  worked 
horse  will  produce  from  45  to  55  pounds  of  excrement  per 
day,  of  which  from  12  to  15  pounds  consists  of  urine. 
The  straw  used  for  bedding  will  amount  to  from  4  to  6 
pounds.    Roberts  has  computed  the  value  of  the  excre- 
ment to  be  nearly  one-half  the  cost  of  the  food,  while 
from  Wolff's  tables,  based  on  a  large  number  of  determi- 
nations in  Europe,  the  combined  solid  and  liquid  excreta 


COMPOSITION   OF   ANIMAL   MANURES  369 

contains  the  following  average  percentages  of  the  organic 
matter,  nitrogen  and  mineral  substances  originally 
present  in  the  food  consumed: 

Per  cent 

Organic  matter 33.90 

Nitrogen 39.50 

Mineral  substances    56.25 

In  Robert's  calculations,  the  value  of  the  manure  is 
based  entirely  upon  its  content  of  nitrogen,  phosphoric 
acid  and  potash,  valued  at  15  cents,  7  cents  and  4.5  cents, 
respectively.  It  is  difficult  to  get  a  true  idea  of  the  value 
of  animal  manure,  as  its  content  of  fertilizing  substances 
is  only  a  part  of  its  manurial  value,  of  which  its  physical 
and  bacteriological  effects  upon  the  soil  are  extremely 
important. 

Horse  manure  has  the  fibrous  matter  of  the  food  less 
well  broken  down  than  has  cow  manure,  and  this,  with 
its  lower  water  content,  produces  a  light,  easily  ferment- 
able substance  that  readily  loses  its  nitrogen,  which 
passes  off  as  ammonium  carbonate.  The  dry  fermen- 
tation, indicated  by  a  whitish  appearance  of  the  interior 
of  the  manure  heap  and  a  slight  smoke,  is  the  cause  of 
this  loss.  The  values  calculated  for  the  excrement  are 
never  realized  in  practice  because  of  the  losses  that 
occur  between  the  stable  and  the  field.  To  preserve 
horse  manure  to  the  best  advantage,  it  should  be  mixed 
with  cow  manure, — the  wet,  compact  character  of 
which  lessens  the  amount  of  fermentation  by  changing 
the  physical  condition  of  the  manure. 

239.  Cow  manure. — A  mature  cow,  given  good  feed, 
will  produce  from  60  to  90  pounds  of  excrement  daily, 


370          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

depending  upon  the  weight  of  the  animal.  Of  this 
20  to  35  pounds  is  likely  to  be  urine.  Even  the  solid 
excreta  contains  a  large  percentage  of  water,  and,  accord- 
ing to  Boussingault,  only  about  one-eighth  of  the  total 
excreta  is  dry  matter. 

The  very  watery  nature  of  cow  excreta  causes  it  to 
require  a  large  amount  of  litter.  In  spite  of  the  lighten- 
ing effect  of  the  litter,  it  decomposes  slowly  as  compared 
with  other  manures,  When  applied  alone  to  the  soil, 
action  is  slow,  but  it  is  prolonged  over  a  considerable 
number  of  years. 

The  loss  of  ammonia  in  the  decomposition  processes 
is  much  less  than  with  horse  manure.  The  admixture 
of  other  manures  adds  much  to  the  rapidity  of  fermen- 
tation and  to  the  ease  of  handling. 

The  percentage  of  organic  matter,  nitrogen  and  min- 
eral substances  contained  in  the  food  of  cattle  that 
appear  ultimately  in  the  excrements  are  as  follows: 

Per  cent 

Organic  matter 27 

Nitrogen 42 

Mineral  matter 50 

This  corresponds  fairly  well  with  the  percentage  for 
horse  manure,  and  would  justify  the  belief  that  the  value 
of  the  manure  would  hold  about  the  same  ratio  to  that 
of  the  food  as  in  the  case  of  the  horse. 

240.  Swine  manure. — The  quantity  of.  excrement 
voided  by  swine  varies  greatly  even  for  mature  animals, 
the  amounts  per  1,000  pounds  live  weight  varying  from 
less  than  50  to  more  than  100  pounds  per  day.  A  more 
concentrated  ration  produces  less  excreta,  but  causes 


COMPOSITION  OF  ANIMAL   MANURES  371 

it  to  be  much  richer  in  fertilizing  ingredients.  Roberts 
calculated  the  value  of  the  manure  produced  in  one 
year  by  a  150-pound  pig  fed  on  a  highly  nitrogenous 
ration  to  be  $3.24,  and  that  of  a  pig  of  similar  weight 
fed  on  a  carbonaceous  ration  to  be  $1.84  for  the  same 
period. 

The  manure  of  swine  is  wet,  but  not  quite  so  much  so 
as  cow  manure.  According  to  Boussingault,  about  one- 
sixth  of  the  solid  excrement  is  dry  matter.  It  decomposes 
slowly.  As  the  urine  contains  by  far  the  larger  part  of 
the  nitrogen,  it  should  be  saved. 

241.  Sheep    manure. — The   total    amount    of   excre- 
ments voided  by  mature  sheep  is  from  30  to  40  pounds 
per  1,000  pounds  of  live  weight,  of  which  about  one- 
fourth  is  dry  mutter.    Although  drier  than  horse  manure 
and  generally  richer  in  nitrogen  it  is  less  likely  to  lose 
that  constituent  by  fermentation,  as  the  compact  nature 
«f  the  solid  excreta  is  not  so  favorable  to  rapid  decom- 
position as  is  the  physical  structure  of  horse  manure. 
It  is  however,  when  placed  in  the  soil,  a  rciulily  acting 
manure  and  is  frequently  used  by  gardeners  for  that 
reason.    To  obtain  the  best  results,  it  should  be  mixed 
with  horse  and  cow  manure. 

242.  Relative  values  of  animal  manures. — Extensive 
experiments   conducted   by    Roberts,    Wing   and   Cava- 
naugh  at  Cornell  University  Experiment  .Station,  with  sev- 
eral different  kinds  of  animals  fed  on  the  common  Ameri- 
can feeds,  but  perhaps  in  somewhat  heavier  rations  than 
the  average,  and  kept  under  normal  conditions,  may  well 
be  taken  to  show  the  relative  values  of  animal  manures, 
although  the  absolute  values  may  be  somewhat  above 


372 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


the  average.  In  calculating  the  values  of  the  manures 
produced  by  these  animals,  nitrogen  is  reckoned  at 
fifteen  cents  per  pound,  phosphoric  acid  at  six  cents, 
and  potash  at  four  and  one-half  cents.  The  composition, 
amount  and  value  of  the  manures  without  litter  are 
given  in  the  following  table. 

TABLE  LV 

COMPOSITION,  AMOUNT  AND  VALUE  OP  MANURES  (WITHOUT 
LITTER)  FROM  DIFFERENT  ANIMALS 


Percentage  compo- 
sition 

Pounds  ingredi- 
ents per  ton 

c 
p 

Production 
per  1,000 
pounds  live 

manure 

weight 

Kinds  of 

¥ 

live  stock 

c 

C 
0 

M 

« 

t> 
o 

M 

0. 

9 
3 

•3d 

H 

0) 

$ 

d/r" 

a 

I 

~3 

1-3 

3  ^ 

>5 

•5 

o 

J« 

K    H 

o 

> 

3  u 

"3  h 

f 

Z, 

O 

CH 

2 

o 

J3 

5 

PH 

h& 

>a 

Horses 

48.70 

0.49 

0.26 

0.48 

9.00 

5.20 

9.60 

$2.21 

48.8 

$27.74 

Cows  .  . 

75.25 

0.43 

0.29 

0.44 

8.60 

5.80 

8.80 

2.02  74.1 

29.27 

Calves  . 

77.73 

0.50 

0.17 

0.53 

10.00 

3.40 

10.60 

2.18  67.8 

24.45 

Swine.  . 

74.13 

0.84 

0.39 

0.32 

16.80 

7.80 

6.40 

3.29  83.6 

60.88 

Sheep.  . 

5952 

0.77 

0.59 

15.40 

7.60 

11.80 

3.30 

34.1  j    26.09 

243.  Poultry  manure. — The  droppings  of  poultry 
are  nearly  twice  as  valuable,  pound  for  pound,  as  cow 
manure,  when  calculated  on  the  value  of  the  nitrogen, 
phosphoric  acid  and  potash  they  contain.  It  is  in  the 
former  constituent  particularly  that  poultry  manure  is 
rich.  A  thousand  pounds  live  weight  of  fowls  produce 
from  thirty  to  forty  pounds  of  droppings  daily.  These 
contain  when  fresh  between  50  and  60  per  cent  of  water 
and  over  1  per  cent  of  nitrogen.  The  nitrogen  is  largely 
present  as  ammonium  compounds.  It  quickly  undergoes 
fermentation,  with  loss  of  nitrogen.  Lime  or  alkalies 


COMPOSITION   OF   ANIMAL   MANURES  373 

decompose  the  ammonium  compounds  with  liberation 
and  loss  of  free  ammonia.  An  absorbent,  such  as  land 
plaster,  superphosphate,  kainit  or  dry  earth  will  greatly 
lessen  the  loss  of  nitrogen.  Mixing  it  with  other  manures 
is  also  advisable. 

When  applied  to  the  soil,  poultry  manure  decom- 
poses rapidly,  and  is  used  by  market  gardeners  on  account 
of  its  rapid  action. 

244.  Factors  affecting  the  values  of  farm  manures. — 
The  value  of  animal  excrements  for  manurial  purposes 
depends  upon  a  number  of  factors,  among  which  are: 
(1)  The   relative   proportions   of   solid   excrement    and 
urine.    (2)  The  species  of  animal  producing  the  manure. 
(3)  The  age  of  the  animal.   (4)  The  character  of  the  food 
the  animal  receives.    (5)  The  use  to  which  the  animal 
is    being    put.      In    addition    to    the    factors    affecting 
the  excrement,   the   manure   may    always   be    modified 
by   the  litter   or    other    absorbent    added,   and    by  the 
method  of   handling.     The   effects   of   solid    and    liquid 
excreta,  and  of  the  species  of  animal,  have  already  been 
discussed. 

245.  Age  of  animal. — A  young  and  growing  animal 
requires    more   nitrogen    and    phosphoric    acid   to   build 
bone  and  muscle  than  does  an  animal  that  has  completed 
its  growth.   This  is  taken  from  the  food,  and  not  excreted 
in  the  urine  or  other  excretory  products,  and  hence  does 
not  appear  in  the  manure. 

246.  Food  of  the  animal. — Since  the  large  part  of  the 
nitrogen   phosphorus   and   potassium   contained   in    the 
food  is  contained  in  either  the  solid  or  liquid  excrement, 
it  follows  that  the  richer  the  food  in  these  constituents 


374 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


the  more  of  them  the  manure  will  contain.  A  highly 
carbonaceous  ration  produces  a  poor  manure  largely 
because  it  is  low  in  nitrogen.  The  manurial  value  of  a 
food-stuff  is  generally  increased  by  passing  through  the 
animal,  provided  it  can  largely  be  recovered,  because 
the  digestion  process  leaves  it  in  a  condition  more  favor- 
able to  decomposition  and  to  thorough  mixing  with  the 
soil. 

247.  Use  of  the  animal. — The  amounts  of  the  ferti- 
lizing constituents  recovered  in  the  excrement  vary  to 
some  extent  with  the  use  that  is  being  made  of  the  ani- 
mal. Animals  that  are  being  fattened,  or  that  are  pro- 
ducing milk,  divert  a  portion  of  the  fertilizing  constit- 
uents to  their  products.  Experiments  by  Laws  and  Gil- 
bert with  different  classes  of  animals  used  for  different 
purposes  show  the  following  disposition  of  some  of  the 
constituents  of  the  food.  As  the  excrements  include 
the  perspiration,  the  small  amount  of  matter  passing 
off  in  that  form  is,  of  course,  not  recovered  in  the  manure. 

TABLE   LVI 


Nitrogen                       Mineral  matter 

Contained 
in 
product 

Contained 
in 
excrement 

Contained 
in 
product 

Contained 
in 
excrement 

Horse  at  rest  

Per  cent 

None 
None 
24.5 
3.9 
14.7 
4.3 

Per  cent 
100.0 
100.0 
75.0 
96.1 
85.3 
95.7 

Per  cent 

None 
None 
10.3 
2.3 
4.0 
3.8 

Per  cent 
100.0 
100.0 
89.7 
97.7 
96.0 
96.2 

Horse  at  work  

Milking  cows    

Fattening  oxen    

Fattening  pigs  

Fattening  sheep  

DETERIORATION   OF   MANURE  375 

It  will  be  seen  from  these  experiment^  that  milch 
cows  divert  more  of  the  fertilizing  constituents  from  the 
manure  than  do  any  other  class  of  animal,  that  fattening 
pigs  divert  much  more  of  the  nitrogen  than  do  cattle 
or  sheep  similarly  employed,  and  that  the  work  of  the 
horse  does  not  affect  the  composition  of  the  manure. 

248.  Deterioration  of  farm  manure. — There  is  always 
a  loss  in  the  value  of  farm  manure  on  standing.   The  two 
processes  most  operative  in  bringing  this  change  about 
are:  (1)  Fermentation.      (2)   Leaching.      The     first     of 
these  is  a  natural  process,  common  to  all  farm  manure, 
and  not  occasioned  by  any  outside  agencies;  the  second 
is  due  to  the  running  off  of  the  liquid  portion  of  the 
manure,  and  to  the  exposure  of  the  manure  to  rain. 

249.  Fermentations. — The    fermentations    occurring 
in  heaps  of  farm  manure  are  produced  both  by  aerobic 
and  anaerobic  bacteria,  that  is,  by  bacteria  requiring 
oxygen  for  their  activity,  and  by  those  that  do  not.   The 
fermentations  of  the  outside  of  the  heap  are  constantly 
different  from  those  on  the  interior,  where  air  does  not 
readily  penetrate;  but,  as  fresh  manure  is  thrown  upon 
the  pile  from  day  to  day,  most  of  the  manure  first  under- 
goes aerobic  fermentation  before  the  anaerobic  bacteria 
begin  their  work. 

It  is  through  the  action  of  bacteria  on  the  nitrogen- 
ous compounds  of  the  manure  that  loss  of  value  through 
fermentations  occurs.  The  action  of  the  aerobic  bacteria 
is  to  convert  the  nitrogen  of  the  organic  matter  into 
ammonia,  which,  owing  to  the  large  formation  of  carbon 
dioxid,  is  partly  converted  into  ammonium  carbonate. 
Both  of  these  substances  being  volatile,  there  is  danger 


376         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

of  their  pasging  off  from  the  heap  into  the  air.   The  drier 
the  heap,  the  more  apt  these  substances  are  to  escape. 

The  production  of  ammonia  is  very  rapid  from  some 
of  the  compounds  in  farm  manure.  Urea,  in  which  form 
the  nitrogen  of  urine  is  largely  found,  undergoes  con- 
version into  ammonia  very  rapidly,  and  some  loss  in 
this  way  is  inevitable  even  under  the  best  management. 
Chemically  the  process  is  a  simple  one,  which  may  be 
represented  by  the  following  equation: 

CON2H4  +  H2O  =  2  NH3  +  CO2. 
2  NH,+CO3  +  H2O  =  (NH4),  CO,. 


The  use  of  certain  preservatives  makes  it  possible 
to  decrease  the  loss  of  ammonia  from  manure.  The 
preservatives  are  intended  to  convert  the  ammonia  into 
a  less  volatile  compound.  For  this  purpose  gypsum, 
kainit,  superphosphates  and  ground  phosphate  rock  are 
used.  The  action  of  gypsum,  for  instance,  in  the  manure, 
is  to  convert  ammonia  or  ammonium  carbonate  into 
the  form  of  ammonium  sulfate,  which  is  not  volatile. 
The  reaction  is  as  follows: 

(NH4),  CO3  +  CaSO4  =  (NH4)2SO4  +  CaCO,. 

It  is  customary  to  sprinkle  the  preservative  in  the 
stall  of  the  animal,  where  it  comes  in  contact  with  the 
excreta  as  soon  as  they  are  voided.  Salts  of  calcium 
other  than  the  sulfate,  cannot  be  used,  on  account  of 
their  action  in  decomposing  ammonium  salts. 

The  decomposition  of  proteins  forming,  among  other 
products,  hydrogen  sulphide,  which  becomes  oxidized 
to  sulfuric  acid,  causes  a  part  of  the  ammonia  to  natu- 


WASTE   OF   MANURE   BY    LEACHING  377 

rally  take  the  form  of  a  sulfate,  which  protects  this  por- 
tion from  volatilization. 

The  other  fermentation  resulting  in  the  loss  of  nitro- 
gen is  due  to  the  action  of  certain  anaerobic  bacteria 
that  convert  ammonium  salts  into  free  nitrogen.  Certain 
of  these  organisms  are  able  to  reduce  nitrates  to  nitrites, 
and  the  latter  to  ammonia,  but  the  greatest  loss  is  doubt- 
less due  to  the  ammonium  salts  formed  directly  from 
proteins.  This  process  occurs  only  in  the  poorly  aerated 
portions  of  the  heap.  There  does  not  appear  to  be  as 
great  loss  of  nitrogen  through  the  action  of  the  anaerobic 
ferments  as  through  the  loss  of  ammonia,  which  makes 
it  advisable,  in  practice,  to  keep  the  manure  heap  as 
compact  as  possible,  and  to  prevent  the  heap  from  be- 
coming very  dry  by  the  application  of  water  in  amounts 
sufficient  to  keep  the  heap  moderately  moist  without 
leaching  it.  In  the  arid  and  semi-arid  parts  of  the  coun- 
try, this  is  an  important  precaution  to  be  taken  in  the 
preservation  of  farm  manure. 

250.  Leaching. — When  water  is  allowed  to  soak 
through  a  manure  heap  and  to  drain  away  from  it.  there 
is  carried  off  in  solution  and  in  suspension  a  certain 
quantity  of  organic  and  inorganic  compounds  contain- 
ing nitrogen  as  urea,  other  organic  nitrogen  in  small 
amounts,  ammonium  salts  and  nitrates,  some  phos- 
phorus and  considerable  potassium,  with  other  mineral 
substances  of  less  importance.  The  amount  of  loss  to  the 
manure  in  this  way  may  be  very  great;  and,  without 
doubt,  in  the  humid  portions  of  the  country  leaching 
is  the  greatest  source  of  loss.  Protection  of  manure  from 
the  rain  is  therefore  very  important. 


378 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


Experiments  conducted  by  Roberts  serve  to  show 
the  rate  and  extent  of  deterioration  of  manure  in  a  region 
having  a  rainfall  of  about  twenty-eight  inches  in  the  six 
months  from  spring  until  autumn,  during  which  period 
the  tests  were  made.  The  loss  arising  from  fermentation 
and  leaching  combined  was  determined  in  these  experi- 
ments. 

Horse  manure  was  lightly  packed  in  a  wooden  box, 
not  water-tight,  surrounded  with  manure,  and  left 
exposed  to  the  weather  from  March  30  to  September  30. 
Analyses  made  at  the  beginning  of  and  at  the  end  of  the 
experiment  showed  the  following: 

TABLE  LVII 


April  25 

September  30 

Loss 

Gross  weight  

Pounds 
4,000.00 

Pounds 
1,730.00 

Per  cent 
57 

Nitrogen  

19.60 

7.79 

60 

Phosphoric  acid  

14.80 

7.79 

47 

Potash  

36.00 

8.65 

76 

At  the  same  time,  cow  manure  was  similarly  treated, 
except  that  300  pounds  of  gypsum  were  mixed  with  it. 
This,  doubtless,  protected  some  of  the  nitrogen,  and  the 
greater  body  of  material  would  also  decrease  loss  of  all 
constituents. 

TABLE  LVIII 


April  25 

September  30 

Loss 

Gross  weight  

Pounds 
10,000 

Pounds 
5,125 

Per  cent 
49 

Nitrogen  

47 

28 

41 

Phosphoric  acid  

32 

26 

19 

Potash  

48 

44 

8 

HANDLING   MANURE  379 

The  greater  loss  suffered  by  the  horse  manure  was 
doubtless  due  in  part  to  the  more  rapid  fermentation 
accompanied  by  volatilization  of  ammonia,  and  to  its 
less  compact  nature  making  it  more  permeable  to  the 
rain  water. 

Roberts  also  reports  an  experiment  in  which  a  block 
of  undisturbed  manure  one  foot  deep,  consisting  of  both 
horse  and  cow  excrement  mixed  with  straw  and  solidly 
packed  by  trampling  of  animals  in  a  covered  shed, 
was  exposed  from  March  31  to  September  30  in  a  gal- 
vanized iron  pan  with  perforated  bottom.  The  losses 
were  as  follows:  Ix)M 

Per    cent 

Nitrogen 3.2 

Phosphoric  acid 4.7 

Potash oo.O 

This  shows  a  great  saving  to  both  kinds  of  manure 
when  they  are  mixed  and  tramped.  The  enormous 
difference  in  the  nitrogen  lost,  without  a  corresponding 
difference  in  the  loss  of  potash,  indicates  that  the  volatili- 
zation of  ammonia,  which  is  greatly  reduced  by  com- 
pacting, is  responsible  for  a  very  large  share  in  the 
deterioration  of  manure,  even  in  a  humid  climate. 

251.  Methods  of  handling. — The  least  opportunity 
lor  deterioration  of  farm  manure  occurs  when  it  is  hauled 
directly  to  the  field  from  the  stall  and  spread  at  once. 
This  is  not  always  possible,  and  manure  must  be  stored 
on  every  farm  for  longer  or  shorter  periods.  In  holding 
manure,  the  two  important  conditions  arc.  a  sufficient, 
but  not  excessive  supply  of  moisture,  and  a  well-com- 
pacted mass.  Water  draining  away  from  a  manure  heap, 


380         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

and  a  fermentation  producing  a  white  appearance  of  the 
manure  under  the  surface  of  the  pile  ("fire  fanging"), 
are  both  sure  indications  of  unnecessary  loss  in  its  ferti- 
lizing value. 

Composting  farm  manure  increases  the  availability 
of  its  fertilizing  constituents;  but,  even  when  carefully 
conducted,  is  accompanied  by  some  loss  of  nitrogen. 
The-  total  amount  of  organic  matter  is  decreased  by 
reason  of  the  decomposition,  in  which  process  carbon 
dioxid  and  water  are  formed,  part  of  which  escapes,  and 
part  remains  in  the  manure.  The  mineral  constituents 
increase  percentagely,  due  to  the  loss  of  organic  matter; 
and  the  water  increases  for  the  same  reason,  and  because 
it  is  sometimes  added  to  the  compost.  The  mineral  con- 
stituents are  not  materially  changed  in  their  solubility, 
but  the  organic  matter  becomes  more  soluble.  The 
nitrogen,  after  conversion  into  ammonium  salts,  is 
oxidized  finally  into  nitrates,  but  only  in  small  amounts, 
and  after  considerable  time.  The  beneficial  effects  of 
composing  are  only  in  small  part  due  to  the  chemical 
changes  in  the  manure,  but  chiefly  to  the  good  physical 
condition  of  the  composted  material,  and  to  the  fact 
that  the  operations  preliminary  to  the  formation  of 
nitrates  have  largely  been  effected  in  the  compost,  and 
when  applied  to  the  soil  nitrification  is  rapid.  Composting 
manure  with  soil,  sod,  muck  or  other  absorbent  material 
increases  the  manurial  value  of  the  latter  by  increasing 
its  decay,  and  therefore  its  availability,  and  by  reducing 
loss  by  leaching. 

The  following  analyses,  by  Voelcker,  show  the  com- 
position of  fresh  and  rotted  farm  manure: 


HANDLING   MANURE  381 

TABLE  LIX 


Fresh 


Rotted 


Water 66.17  75.42 

Soluble  organic  matter j  2.48  3.71 

Soluble  organic  nitrogen 0.15  0.30 

Soluble  inorganic  matter |  1.54  1.47 

Insoluble  organic  matter 25.70  12.82 


Insoluble  inorganic  matter 


4.05 


6.58 


In  applying  farm  manure  to  the  field,  it  is  customary 
either  to  throw  it  from  the  wagon  into  small  heaps,  from 
which  it  is  distributed  later,  or  to  scatter  it  as  evenly 
as  possible  immediately  on  hauling  it  to  the  field.  The 
use  of  the  automatic  manure  spreader  accomplished 
the  latter  procedure  in  an  admirable  manner.  As  be- 
tween these  two  methods,  the  advantage,  so  far  as  the 
conservation  of  the  manurial  value  is  concerned,  is 
with  the  practice  of  spreading  immediately.  When  piled 
in  small  heaps,  fermentation  goes  on  under  conditions 
that  cannot  be  controlled,  and  that  may  be  very  unfavor- 
able. The  heaps  may  dry  out,  and  thus  lose  much  of 
their  nitrogen;  or  they  are  likely  to  leave  the  field  un- 
evenly fertilized  by  leaching  into  the  soil  directly  under 
and  adjacent  to  the  heap.  On  the  other  hand,  when 
spread  immediately,  little  fermentation  takes  place, 
as  the  temperature  is  generally  low  and  the  soluble 
compounds  are  leached  quite  uniformly  into  the  soil. 
Plowing  should  follow  as  closely  as  possible  the  spread- 
ing of  the  manure,  and,  except  in  winter,  at  which  time 
deterioration  is  not  likely  to  be  great,  this  can  well  be 
done. 


382         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

The  amounts  and  frequency  with  which  farm  manure 
should  be  applied  must  depend,  to  some  extent,  upon  the 
nature  of  the  farming  and  upon  the  character  of  the  soil. 
Farm  manure  tends  to  render  all  soils  more  porous  and 


FIG.  107.     The  wrong  way  to  distribute  manure.   There  is  large  loss  by  decay 
and  an  uneven  growth  of  crop. 

light.  A  naturally  light  soil  may  be  rendered  less  pro- 
ductive by  the  application  of  heavy  dressings  of  manure; 
particularly  in  a  dry  climate  is  this  the  case.  In  regions 
where  so-called  "dry  farming"  is  practiced,  the  return 
of  organic  matter  to  the  soil  is  a  great  problem,  on 
account  of  the  difficulty  in  accomplishing  its  decay 


PLACE   FOR   MANURE  383 

when  plowed  under.  Composting,  or  plowing  under  after 
it  has  been  applied  to  sod  for  several  months,  or  incorpo- 
rating with  a  green  manure,  are  methods  that  must  be 
used  with  "dry  farming." 

Even  on  heavy  soils  in  a  humid  region,  there  is  an 
advantage  in  applying  small  dressings  of  farm  manure 
frequently,  rather  than  large  amounts  at  long  intervals. 
Organic  matter  decomposes  more  rapidly  when  present 
in  the  soil  in  relatively  small  amounts,  and  its  influence 
on  the  solubility  of  plant  nutrients  is  therefore  greater 
in  proportion  to  the  amount  of  manure  used.  There  can 
be  no  doubt  that  the  bacterial  flora  introduced  into  the 
soil  by  the  incorporation  of  farm  manure  is  an  important 
factor  in  its  usefulness,  and  when  this  occurs  at  frequent 
intervals  it  has  a  marked  effect  on  productiveness. 
Applications  of  ten  tons  to  the  acre  are  better  than 
twenty  tons  at  twice  the  interval. 

252.  Place  in  crop  rotation. — When  a  crop  rotation 
includes  grass  or  clover  as  one  of  the  courses,  the  appli- 
cation of  farm  manure  may  well  be  made  at  that  time 
as  a  top-dressing.  The  spreading  can  be  done  at  times 
when  cultivated  land  would  not  be  accessible,  and  the 
crop  of  hay  will  profit  greatly.  The  sod,  when  plowed, 
is  frequently  planted  to  corn — a  crop  that  is  rarely 
injured  by  farm  manure.  On  light,  dry  soils  this  practice 
is  of  advantage,  as  already  explained. 

Most  cultivated  crops,  with  the  exception  of  tobacco, 
and  occasionally  sugar-beets,  are  much  benefited  by 
farm  manure.  Small  grains  are  usually  benefited  when 
grown  on  poor,  heavy  soils  with  plenty  of  rainfall:  but 
in  a  dry  region  farm  manure  should  not  be  applied 


384         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

for  these  crops,  and  on  rich  soils  manure  is  likely  to  cause 
small  grain  to  lodge. 

Farm  manure,  in  judicious  amounts,  may  be  plowed 
under  in  orchards  to  great  advantage. 

263.  Functions. — The  useful  function  which  farm 
manures  perform  in  the  soil  are  as  follows:  (1)  To 
improve  the  physical  condition  of  the  soil  by  the  intro- 
duction of  organic  matter,  with  its  favorable  influence 
on  the  structure  and  moisture  content.  (See  page  129.) 
(2)  To  add  a  certain  quantity  of  plant-food  in  a  compara- 
tively readily  available  condition.  (3)  To  introduce 
a  new  bacterial  flora  capable  of  increasing  the  rapidity 
of  decomposition  of  organic  matter,  and  of  thereby  in- 
creasing the  amount  of  available  fertility. 

254.  Green  Manures. — Crops  that  are  grown  only  for 
the  purpose  of  being  plowed  under  to  improve  the  soil 
are  called  green  manures.  They  may  benefit  the  soil 
in  one  or  all  of  four  ways:  (1)  By  utilizing  soluble  plant- 
food  that  would  otherwise  escape  from  the  soil.  (2)  By 
incorporating  vegetable  matter  with  the  soil.  (3)  Le- 
guminous crops,  when  used,  add  to  the  nitrogen  content 
of  the  soil  through  the  fixation  of  atmospheric  nitrogen. 
(4)  Plant-food  from  the  lower  soil  may  be  brought  to  the 
surface  soil. 

A  large  number  of  crops  may  be  used  for  this  purpose, 
but  certain  ones  are  more  useful  than  others,  while  the 
climate  determines  to  some  extent  which  crops,  should 
be  used.  Leguminous  crops  have  the  great  advantage 
of  acquiring  nitrogen  from  the  air.  Crops  that  can  be 
planted  in  the  fall  and  grow  during  the  cool  weather 
can  be  utilized  when  otherwise  the  land  would  frequently 


GREEN  MANURES  385 

lie  bare.  Deep-rooted  crops  usually  accumulate  a  large 
amount  of  nutriment  from  the  soil,  and  considerable 
from  the  lower  depths.  They  are  therefore  useful  in 
bringing  plant-food  to  the  upper  layer  of  soil.  Succulent 
crops  decompose  easily,  and  dry  out  the  soil  less,  when 
plowed  under,  than  do  woody  crops.  Crops  with  exten- 
sive root-systems  prevent  loss  of  soluble  matter  more 
thoroughly  than  do  plants  with  small  roots. 

265.  Leguminous  crops. — A  soil  that  has  become  less 
productive  under  cultivation,  and  that  must  be  improved 
before  profitable  crops  can  be  grown,  receives  more 
benefit  from  the  use  of  leguminous  crops  than  any 
other.  The  legume  to  use  is  naturally  the  one  best 
adapted  to  the  region  in  which  the  soil  is  located.  Red 
clover,  mammoth  clover  and  field  peas  on  the  soils  to 
which  they  are  adapted  in  the  northern  states;  alsike 
clover  in  the  wet  soils  of  that  region;  cowpeas  and  crim- 
son clover  in  the  South,  and  alfalfa,  clovers,  soy  beans 
and  cow  peas  in  the  West,  are  the  principal  leguminous 
green-manuring  crops.  More  recently  a  positive  effort 
has  been  made  in  certain  northern  states  to  grow  sweet 
clover  (Melilotus  alba),  which  is  a  vigorous  wild  legume, 
as  a  green  manure  crop.  Marked  success  has  followed 
its  use,  but,  like  alfalfa  and  the  clovers,  it  requires  a  soil 
well  stocked  with  lime. 

The  legumes  have  the  important  property  of  securing 
nitrogen  from  the  air,  which  is  added  to  the  soil  from 
the  decomposition  of  the  tops  and  roots  when  the  crop 
is  plowed  under.  The  nitrogen  contained  in  a  ton  of  the 
green  crop,  when  in  a  condition  to  plow  under,  is  as 
follows: 


386 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


TABLE  LX 


Nitrogen 
per  ton 

Probable 
yield  per 
acre 

Nitrogen 
per  acre 

Red  or  mammoth  clover  

Pounds 
10 

Tons 
6 

Pounds 
60 

Crimson  clover  

9 

6 

54 

Alsike  clover   

10 

5 

50 

Alfalfa  

14 

8 

112 

Cowpeas  

8 

6 

48 

Soy  beans  

10 

6 

60 

Field  peas  

11 

5 

55 

Not  all  of  the  nitrogen  contained  in  these  crops  is 
taken  from  the  air.  On  soils  rich  in  nitrogen,  a  consider- 
able proportion  may  be  obtained  from  the  soil.  On  poor 
soils,  the  proportion  derived  from  the  atmosphere  is 
considerably  larger.  The  soils  needing  the  nitrogen 
most  are  those  that  benefit  most  largely. 

As  the  legumes  need  other  fertilizing  material  in  an 
available  form  to  produce  a  good  yield,  mineral  ferti- 
lizers or  farm  manure  should  be  added  to  the  soil. 
Especially  on  run-down  land  this  treatment  is  profitable. 

The  crops  should  be  plowed  under,  while  green  and 
succulent,  as  they  decompose  most  readily  at  that  stage. 
On  sandy  soils  and  in  dry  regions,  the  soil  may  be 
rendered  so  porous  by  plowing  under  a  crop  of  dry 
vegetation  that  the  capillary  rise  of  water  is  greatly 
decreased,  and  the  movement  of  air  through  the  soil 
causes  it  to  become  very  dry. 

The  perennial  clovers  (red,  mammoth  and  alsike) 
and  alfalfa  do  ,not  make  a  rapid  growth  after  seeding, 
which  is  a  disadvantage  when  quick  results  are  desired, 


COVER   CROPS   AND  GREEN   MANURE  387 

as  on  a  badly  run-down  soil.  Crimson  clover  is  an  annual, 
and  in  the  central  and  southern  states  may  be  sown  in 
the  fall  and  plowed  under  in  the  late  spring,  thus  making 
use  of  a  period  of  the  year  when  the  soil  is  most  likely 
to  be  unoccupied  by  a  crop.  Cowpeas,  soy-beans  and 
field  peas  must  be  grown  during  the  summer  months. 
Vetch  promises  to  be  a  useful  green  manure  for  winter 
growth  in  the  northern  states. 

256.  Cereal  crops. — Where  it  is  desired  to  keep  a 
crop  on  the  soil  during  the  autumn,  winter  and  spring, 
for  the  purpose  of  utilizing  the  soluble  plant-food,  the 
cereals,  especially  rye,  are  useful.  Rye  has  the  advan- 
tage of  being  an  inexpensive  crop  to  seed,  besides  being 
very  hardy,  and  capable  of  growing  on  poor  soil.  It 
furnishes  fall  pasture,  but  should  not  be  pastured  in  the 
spring  if  intended  for  green  manure.  It  is  important 
that  it  be  plowed  under  while  green. 

Buckwheat,  on  account  of  its  ability  to  grow  on  poor 
soil,  is  adapted  to  use  as  a  green  manure,  but  it  must 
be  grown  in  the  summer. 


D.    ORGANISMS  IN  THE  SOIL 

A  vast  number  of  organisms,  animal  and  vegetable, 
live  in  the  soil.  By  far  the  greater  part  of  these  belong 
to  plant  life,  and  these  comprise  the  forms  of  greatest 
effect  in  producing  those  changes  in  structure  and 
composition  which  contribute  to  soil  productiveness. 
Most  of  the  organisms  are  so  minute  as  to  be  seen  only 
by  the  aid  of  the  microscope,  while  a  much  smaller 
proportion  range  from  these  to  the  size  of  the  larger 
rodents.  They  may  thus  be  classed  as  macro-organ- 
isms and  micro-organisms. 

I.    MACRO-ORGANISMS  OF  THE  SOIL 

Of  the  macro-organisms  in  the  soil  the  animal 
forms  belong  chiefly  to  (1)  rodents,  (2)  worms,  (3) 
insects;  and  the  plant  forms  to  (1)  the  large  fungi  and 
(2)  plant  roots. 

267.  Rodents. — The  burrowing  habits  of  rodents, 
of  which  the  ground-squirrel,  mole,  gopher  and  prairie- 
dog  are  familiar  examples,  result  in  the  pulverization 
and  transfer  of  very  considerable  quantities  of  soil. 
While  their  activities  are  often  not  favorable  to  agri- 
culture, the  effect  upon  the  character  of  the  soil  is 
quite  beneficial,  and  analogous  to  that  of  good  tillage. 
Their  burrows  also  serve  to  aerate  and  drain  the  soil, 
and  in  permanent  pastures  and  meadows  are  of  much 
value  in  this  way. 

(388) 


SOIL  MICRO-ORGANISMS  389 

268.  Worms. — The  common  earthworm  is  the  most 
conspicuous  example  of  the  benefit  that  may  accrue 
from  this  form  of  life.  Darwin,  as  the  result  of  care- 
ful measurements,  states  that  the  amount  of  soil 
passed  through  these  creatures  may,  in  a  favorable 
soil  in  a  humid  climate,  amount  to  ten  tons  of  dry 
earth  per  acre  annually.  The  earthworm  obtains  its 
nourishment  from  the  organic  matter  of  the  soil,  but 
takes  into  its  alimentary  canal  the  inorganic  matter 
as  well,  expelling  the  latter  in  the  form  of  casts 
after  it  has  passed  entirely  through  the  body.  The 
ejected  material  is  to  some  extent  disintegrated, 
and  is  in  a  flocculated  condition.  The  holes  left  in 
the  soil  serve  to  increase  aeration  and  drainage,  and 
the  movements  of  the  worms  bring  about  a  notable 
transportation  of  lower  soil  to  the  surface,  which  aids 
still  more  in  effecting  aeration.  Darwin's  studies 
led  him  to  state  that  from  one-tenth  to  two-tenths 
of  an  inch  of  soil  is  brought  to  the  surface  of  land  in 
which  earthworms  exist  in  normal  numbers. 

Instances  are  on  record  of  land  flooded  for  a  con- 
siderable period  so  that  the  worms  were  destroyed, 
and  the  productiveness  of  the  soil  was  seriously 
impaired  until  it  was  restocked  with  earth-worms. 

Wollny  conducted  experiments  with  soil,  in  one 
case  containing  earthworms,  and  in  another  destitute 
of  them.  Although  there  was  much  variation  in  his 
results,  they  were  in  every  case  in  favor  of  the  soil 
containing  the  worms,  and,  in  a  number  of  the  tests, 
the  yield  on  rich  soil  was  several  times  as  great  as  where 
no  worms  were  present. 


390         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

Earthworms  naturally  seek  a  heavy,  compact  soil, 
and  it  is  in  soil  of  this  character  that  they  are  most 
needed,  on  account  of  the  stirring  and  aeration  they 
effect.  Sandy  soil  and  the  soils  of  the  arid  regions, 
in  which  are  found  few  or  no  earthworms,  are  not 
usually  in  need  of  their  activities. 

259.  Insects. — There  is  a  less  definite,  and  probably 
less  effective,   action  of  a  similar  kind  produced  by 
insects.    Ants,  beetles,  and  the  myriads  of  other  bur- 
rowing insects  and  their  larvae  effect  a  considerable 
movement  of  soil  particles,  with  a  consequent  aeration 
of  the  soil.    At  the  same  time  they  incorporate  in  the 
soil  a  considerable  amount  of  organic  matter. 

260.  Large  fungi. — The  larger  fungi  are  chiefly  con- 
cerned in  bringing  about  the  first  stages  in  the  decom- 
position   of    woody    matter,    which    is    disintegrated 
through  the  growth  in  its  tissues  of  the  root-mycelia 
of  the  fungi.    These  break  down  the  structure,   and 
thus  greatly  facilitate  the  work  of  the  decay  bacteria. 
Action  of  this  kind  is  largely  confined  to  the  forest 
and  is  not  of  much  importance  in  cultivated  soil. 

Another  function  of  the  large  fungi  is  exercised 
in  the  intimate  and  possibly  symbiotic  relation  of  the 
fungal  hyphae  to  the  roots  of  many  forest  trees,  in 
soil  where  nitrification  proceeds  very  slowly,  if  at  all, 
for  nitrates  are  apparently  never  present  in  forest 
soils.  This  enveloping  system  of  hyphse,  which  may 
consist  of  masses  in  a  definite  zone  of  the  cortex, 
with  occasional  filaments  passing  outward  into  the 
soil,  or  which  may  surround  the  root  with  a  dense 
mass  of  interwoven  hyphae,  is  called  mycorhiza. 


SOIL   MICRO-ORGANISMS  391 

The  cereal,  cruciferous,  leguminous  and  solanaceous 
plants  are  not  associated  with  mycorhiza.  Mycotropic 
plants  are  usually  those  that  live  in  a  humus  soil 
filled  with  the  mycelia  of  fungi.  It  is  thought  that 
the  mycorhiza  aid  the  higher  plants  to  obtain  nutri- 
ment that  they  must  strive  for  in  competition  with 
the  fungi. 

Mycotropic  plants  are  also  able  to  grow  with  a  very 
small  transpiration  of  moisture,  as  is  well  known  to 
be  the  case  with  many  conifers;  and  this  restricted 
transpiration  would  doubtless  result  in  lack  of  nutri- 
ment were  it  not  for  the  assistance  of  the  mycorhiza. 

261.  Plant  roots. — The  roots  of  plants  assist  in  pro- 
moting productiveness  of  the  soil  both  by  contributing 
organic  matter  and  by  leaving,  upon  their  decay, 
openings  which  render  the  soil  more  permeable  to 
water  and  which  also  facilitate  drainage  and  aeration. 
The  dense  mass  of  rootlets,  with  their  minute  hairs 
that  are  left  in  the  soil  after  every  harvest,  furnish 
a  well-distributed  supply  of  organic  manure,  which  is 
not  confined  to  the  furrow  slice,  as  is  artificially  incor- 
porated manure.  The  drainage  and  aeration  of  the 
lower  soil,  due  to  the  openings  left  by  the  decomposed 
roots,  are  of  the  greatest  importance  in  heavy  soil, 
and  the  beneficial  effects  of  clover  and  other  deep- 
rooted  plants  are  due  in  no  small  measure  to  this 
function. 

II.     MICKO-OKGANISMS  OF  THK  SOIL 

Of  the  micro-organisms  commonly  existing  in 
soils,  the  great  majority  belong  to  plant  rather  than 


392 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


FIG.  108. 

Nematodes    enter- 
ing a  root. 


to  animal  life.  Of  the  latter,  the  only  organisms  of 
economical  importance  are  the  nematodes,  whose 
injurious  effect  upon  plant  growth  is  accomplished 
through  the  formation  of  galls  on  the 
roots,  in  which  the  young  are  hatched 
and  live  to  sexual  maturity. 

262.  Plant      micro-organisms.— The 
microscopic   plants  of  the  soil   may   be 
classed   as    slime-molds,  bacteria,   fungi 
and  algae. 

263.  Plant  micro-organisms  injurious 
to      higher      plants.  —  Injurious      plant 
micro-organisms    are    confined     mostly 

to  fungi  and  bacteria.  They  may  be  entirely  para- 
sitic in  their  habits,  or  only  partially  so.  They 
injure  plants  by  attacking  the  roots.  Those  attacking 
other  portions  of  plants  may  live  in  the  soil  during 
their  spore  stage,  but  these  are  not  strictly  micro- 
organisms of  the  soil.  Some  of  the  more  common  dis- 
eases produced  by  soil  organisms  are:  Wilt  of  cotton, 
cowpeas,  watermelon,  flax,  tobacco,  tomatoes,  etc., 
damping-off  of  a  large  number  of  plants,  root-rot, 
galls,  etc. 

These  fungi  or  bacteria  may  live  for  long  periods, 
probably  indefinitely,  in  the  soil,  if  the  conditions 
necessary  for  their  growth  are  maintained.  Some  of 
them  will  die  within  a  few  years  if  their  host  plants 
are  not  grown  upon  the  soil,  but  others  are  able  to 
maintain  existence  on  almost  any  organic  substance. 
Once  a  soil  is  infected,  it  is  likely  to  remain  so  for  a 
long  time,  or  indeed  indefinitely.  Infection  is  easily 


PLANT   MICRO-ORGANISMS  393 

carried.  Soil  from  infected  fields  may  be  carried  on 
implements,  plants,  rubbish  of  any  kind,  in  soil  used 
for  inoculation  of  leguminous  crops,  or  even  in  stable 
manure  containing  infected  plants,  or  in  the  feces 
resulting  from  the  feeding  of  infected  plants.  Flooding 
of  land  by  which  soil  is  washed  from  one  field  to  another 
may  be  a  means  of  infection. 

Prevention  is  the  best  defense  from  diseases  pro- 
duced by  these  soil  organisms.  Once  disease  has  pro- 
cured a  foothold,  it  is  practically  impossible  to  eradi- 
cate all  its  organisms.  Rotation  of  crops  is  effective 
for  some  diseases,  but  entire  absence  of  the  host  crop 
is  more  often  necessary.  The  use  of  lime  is  beneficial 
in  the  case  of  certain  diseases.  Chemicals  of  various 
kinds  have  been  tried  with  little  success.  .Steam- 
sterilization  is  a  practical  method  of  treating  green- 
house soils  for  a  number  of  diseases.  The  breeding  of 
plants  immune  to  the  disease  affecting  its  particular 
species  has  been  successfully  carried  out  in  the  case  of 
the  cowpea  and  cotton  plants  and  can  doubtless  be 
accomplished  with  others. 

264.  Plant  micro-organisms  not  injurious  to  higher 
plants. — The  vegetable  micro-organisms  of  the  soil 
all  take  an  active  part  in  removing  dead  plants  and 
animals  from  the  surface  of  the  soil,  and  in  bringing 
about  the  other  operations  that  are  necessary  for  the 
production  of  plants.  The  first  step  in  the  preparation 
for  plant  growth  is  to  remove  the  remains  of  plants  and 
animals  that  would  otherwise  accumulate,  to  the  ex- 
clusion of  other  plants.  These  are  decomposed  through 
the  action  of  organisms  of  various  kinds,  the  inter- 


394          THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

mediate  and  final  products  of  decomposition  assisting 
plant  production  by  contributing  nitrogen  and  certain 
mineral  compounds  that  are  a  directly  available  source 
of  plant  nutriment,  and  also  by  the  effect  of  certain 
of  the  decomposition  products  upon  the  mineral 
substances  of  the  soil,  by  which  they  are  rendered 
soluble  and  hence  available  to  the  plant. 

Through  these  operations  the  supply  of  carbon  and 
nitrogen  required  for  the  production  of  organic  matter 
is  kept  in  circulation.  The  complex  organic  compounds 
in  the  bodies  of  dead  plants  or  animals,  in  which  con- 
dition plants  cannot  use  them,  are,  under  the  action 
of  micro-organisms,  converted  by  a  number  of  stages 
into  the  very  simple  compounds  used  by  plants.  In 
the  course  of  this  process,  a  part  of  the  nitrogen  is 
sometimes  lost  into  the  air  by  conversion  into  free 
nitrogen,  but  fortunately  this  may  be  recovered  and 
even  more  nitrogen  taken  from  the  air  by  certain  other 
organisms  of  the  soil. 

The  slime  molds,  bacteria,  fungi  and  algae  all  play 
a  part  in  these  processes,  but  none  of  them  so  actively 
during  every  stage  of  the  process  as  do  the  bacteria. 
Molds  and  fungi  are  particularly  active  in  the  early 
stages  of  decomposition  of  both  nitrogenous  and  non- 
nitrogenous  organic  matter.  Molds  are  also  capable 
of  ammonifying  proteins,  and  even  reforming  the 
complex  protein  bodies  from  the  nitrogen  of  ammonium 
salts.  Certain  of  the  molds  and  algae  are  apparently 
able  to  fix  atmospheric  nitrogen,  and  contribute  a 
supply  of  carbohydrates  required  for  the  use  of  the 
nitrogen-fixing  bacteria. 


BACTERIA    OF    THE    SOIL 


395 


265.  Bacteria. — Of  the  several  forms  of  micro- 
organisms found  in  the  soil,  bacteria  are  the  most 
important.  In  fact,  the  abundant  and  continued  growth 
of  plants  upon  the  soil  is  absolutely  dependent  upon 
the  presence  of  bacteria,  as  through  their  action  chemi- 
cal changes  are  brought  about  which  result  in  making 
soluble  both  organic  and  inorganic  material  necessary 

for  the  life  of  higher  plants,  and 
which,  in  part  at  least,  would 
not  otherwise  occur. 

Bacteria  are  thus  transform- 
ers, and  not  producers,  of  fer- 
tility in  the  soil,  although,  as 
we  shall  see  later,  certain  kinds 
of  bacteria  take  nitrogen  from 
the  air  and  leave  it  in  the  soil. 
With  this  exception,  however, 
they  add  no  plant  food  to  the 
soil.  It  is  their  action  in  render- 
ing available  to  the  plant  ma- 
terial already  present  in  the  soil 
that  constitutes  their  greatest 
present  value  in  crop-produc- 
tion. It  is  to  their  activity  in  conveying  nitrogen  from 
the  air  to  the  soil  that  we  are  indebted  for  most  of 
our  supply  of  nitrogen  in  virgin  soils. 

It  is  not  usually  the  entire  absence  of  bacteria 
from  the  soil  that  is  to  be  avoided  in  practice,  for  all 
arable  soils  contain  bacteria,  although  sometimes  not 
all  of  the  desirable  forms;  but.  as  great  bacterial 
activity  is  required  for  the  large  production  of  crops, 


Flo.  109.  Sonic  typos  of 
Boil-bacteria,  highly  magnified, 
a.  Nitrate  i'ormrrs;  l>,  nitritt" 
formrls;  r,  linctrriit  grnvealens; 
<t,  li.  fiutiformix;  r.  It.  neblilia; 
f,  Clo.itiriilnnn  piixlfiiruiniim. 


396          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

the  practical  problem  is  to  maintain  a  condition  of 
soil  most  favorable  to  such  activity. 

266.  Distribution. — Bacteria  are  found  almost  uni- 
versally in  soils,  although  they  are  much  more  numer- 
ous in  some  soils  than  in  others.  A  number  of  investi- 
gators have  stated  that  in  soils  from  different  locali- 
ties and  of  different  types  that  they  have  examined, 
the  numbers  of  bacteria  were  proportional  to  the 
productiveness  of  the  soils.  The  number  of  bacteria 
present  has,  in  some  cases,  been  shown  to  be  propor- 
tional to  the  amount  of  humus  contained  in  the  soil. 
It  is  natural  to  expect  that  within  certain  limits 
both  of  these  findings  will  hold.  The  conditions  ob- 
taining in  a  productive  soil  are  those  favorable  to 
the  development  of  certain  forms  of  bacteria,  and  these 
kinds  constitute  a  very  large  proportion  of  those  gen- 
erally found  in  soils.  However,  there  is  evidence 
that  comparatively  unproductive  soils  may  contain 
a  large  number  of  bacteria  which  are  presumably 
not  favorable  to  plant-growth. 

Samples  of  soil  taken  from  certain  productive 
and  relatively  unproductive  portions  of  a  field  on 
Cornell  University  farm  contained  a  larger  number 
of  bacteria  in  the  poor  soil,  although  the  two  soils 
were  equally  well  drained,  and  the  good  soil  had  slightly 
more  organic  matter.  They  had  also  received  practi- 
cally the  same  treatment  during  the  preceding  few 
years. 

Character  of  Number  of   bacteria 

soil  per  gram  of  dry  soil 

Good 1,200,000 

Poor  . .  1,600,000 


ABUNDANCE   OF   SOIL    BACTERIA  397 

After  wheat  had  been  growing  for  two  months 
on  these  soils  in  the  greenhouse,  and  maintained  at 
the  same  moisture  content,  they  were  again  sampled. 

Character  of  Number  of  bacteria 

soil  per  gram  of  dry  soil 

Good 760,000 

Poor    1 ,120,000 

Another  reason  why  this  relation  between  the 
number  of  bacteria  and  soil  productiveness  does  not 
hold  is  that  those  bacteria  having  the  same  functions 
in  relation  to  plant-food  do  not  always  have  the  same 
physiological  efficiency.  In  other  words,  they  do  not 
have  the  same  virulence,  a  small  number  in  some 
cases  being  able  to  bring  about  the  same  changes  that 
in  other  cases  require  a  much  larger  number. 

Bacteria  are  found  chiefly  in  the  upper  layers  of 
soil,  although  not  at  the  immediate  surface  of  the 
ground.  The  layer  between  the  first  and  sixth  or 
seventh  inches  contains,  in  most  soils,  the  groat  bulk 
of  the  bacteria  present.  Below  that  deptli  they  de- 
crease in  numbers,  and  below  a  depth  of  six  to  eight 
feet  there  are  usually  none. 

267.  Numbers. — The  number  of  bacteria  in  any 
soil  will  naturally  vary  with  the  conditions  that  favor 
or  discourage  their  growth.  In  sandy  soils,  forest 
soils,  desert  soils,  acid  soils,  waterlogged  soils  and 
soils  low  in  humus,  the  bacteria  are  either  absent  or 
very  few  in  numbers.  In  soils  very  rich  in  organic 
matter,  especially  where  animal  manure  has  been 
applied,  or  where  a  carcass  has  been  buried,  the  num- 
ber becomes  very  large,  as  many  as  100.000  000  per 


398 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


gram  having  been  found;  while  in  soil  of  ordinary 
fertility  and  tilth  the  numbers  range  from  1,000,000 
to  5,000,000  per  gram.  The  extreme  rapidity  with 
which  reproduction  occurs  makes  it  possible  for  the 
number  to  increase  enormously  when  conditions  are 
favorable  for  their  growth.  While,  therefore,  very 
few  bacteria  are  present  in  soils  of  the  northern  states 
during  the  winter,  the  number  increases  with  great 
rapidity  in  the  spring.  Marshall  Ward  has  shown 
that  in  the  mild  winters  in  England  some  soil  bac- 
teria at  least  continue  their  activity  throughout  the 
winter.  In  the  southern  states  of  America  the  same 
is  doubtless  true. 

The  following  table  shows  the  number  of  bacteria 
per  gram  of  soil  found  in  different  parts  of  the  United 
States  during  some  portion  of  the  growing  season: 

TABLE  LXI 


State 

Soil 

Crop 

Investi-  • 
gator 

Number 

Delaware  .  . 
Delaware  .  . 
Delaware  .  . 

Delaware  .  . 
Delaware  .  . 
Kansas  .  .  . 

Rich  garden 
Loam 

Grass,  12  yrs. 
Grass,  4  yrs. 
Clover,  follow- 
ing fallow 
Woodland 
Vegetables 

Chester 
Chester 
Chester 

Chester 
Chester 
Mayo  & 

425,000 
425,000 
1,880,000 

70,000 
1,8GO,000 
33,931,747 

Kansas  .... 

(humus  2.19%) 
Loam 

Kinsley 
Mayo  & 

53,596,060 

Kansas  .... 

(humus  3.07%) 
Thin  soil, 

Kinsley 
Mayo  & 

78,534 

Kansas  .... 

gumbo  subsoil 
Loam,  low  in 

Kinsley 
Mayo  & 

8,543,006 

Kansas  .... 

humus 
Loam,  low  in 

Kinsley 
Mayo  & 

3,192,131 

humus 

Kinsley 

SOIL    BACTERIA,    CONDITIONS   FOR   GROWTH       399 

268.  Conditions  affecting  growth. — Many  conditions 
of  the  soil  affect  the  growth  of  bacteria.    Among  the 
most   important   of   these   are   the   supply   of  oxygen 
and  moisture,  the  temperature,  the  presence  of  organic 
matter,  and  the  acidity  or  basicity  of  the  soil. 

269.  Oxygen. — All    soil    bacteria   require   for   their 
growth  a  certain  quantity  of  oxygen.    Some  bacteria, 
however,  can  continue  their  activities  with  much  less 
oxygen  than  can  others.    Those  requiring  an  abundant 
supply  of  oxygen   have  been   called  aerobic   bacteria, 
while  those  preferring  little  or  no  air  are  designated 
anaerobic  bacteria.    This  is  an  important  distinction, 
because  those  bacteria  which  are  of  the  greatest  benefit 
to  the  soil  are,  in  the  main,  aerobes,  and  those  bac- 
teria   that    are    injurious    in    their    action    are    chiefly 
anaerobes.     However,  it  seems  likely  that   an   aerobic 
bacterium    may   gradually   accommodate   itself   within 
certain     limits    to     an    environment     containing    less 
oxygen,  and  an  anaerobic  bacterium  may  accommodate 
itself  to  the  presence  of  a  larger  amount   of  oxygen. 
Thus  a  bacterium  may  be  most  active  in  the  presence 
of  an  abundant  supply  of  oxygen;  but,  when  subjected 
to   conditions   in    which    the   supply    is   small,    growth 
continues,  but  with  lessened  vigor.  The  term  facultative 
bacteria    has    been    used    to    designate    those    bacteria 
that    are    able    to    adapt    themselves    to    considerable 
variation  in  oxygen  supply.    The  structure,  tilth  and 
drainage   of   the   soil    consequently    determine   largely 
whether  aerobic  or  anaerobic   bacteria  shall   be   most 
active. 

270.  Moisture.  —  Bacteria     require     some     moisture 


400          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

for  their  growth.  A  notable  decrease  in  the  moisture 
content  of  the  soil  may  temporarily  decrease  the 
number  of  bacteria  by  limiting  their  development  to 
the  films  of  moisture  surrounding  the  particles.  With 
a  decrease  in  the  moisture  content  of  any  soil,  there 
occurs  an  increase  in  the  oxygen  in  the  interstitial 
spaces.  Those  bacteria  thriving  in  the  presence  of 
oxygen  are  thereby  favored,  and  the  character  of  the 
bacterial  flora  is  correspondingly  changed.  When  the 
soil  remains  saturated,  or  nearly  so,  for  any  consider- 
able period,  the  anaerobic  forms  assert  themselves, 
and  the  usually  beneficial  activities  of  the  aerobic 
bacteria  are  temporarily  suspended.  The  most  favor- 
able moisture  conditions  for  the  activity  of  the  most 
desirable  bacteria  is  that  found  in  a  well-drained  soil. 
271.  Temperature. — Soil  bacteria,  like  other  plants, 
continue  life  and  growth  under  a  considerable  range 
of  temperature.  Freezing,  while  rendering  bacteria 
dormant,  does  not  kill  them,  and  growth  begins  slightly 
above  that  point.  Warrington  has  shown  that  nitri- 
fication goes  on  at  temperatures  as  low  as  37°  to  39° 
Fahr.  It  is  not,  however,  until  the  temperature  is 
considerably  higher  that  the  functions  of  any  of  the 
soil  bacteria  are  pronounced.  From  70°  to  110°  Fahr. 
their  activity  is  greatest,  and  it  diminishes  perceptibly 
below  or  above  those  points.  The  thermal  death  points 
of  most  forms  of  bacteria  is  found  at  some  point 
between  110°  and  160°  Fahr.,  but  the  spore  forms 
even  resist  boiling.  Only  in  some  desert  soils  does  the 
natural  temperature  reach  a  point  sufficiently  high 
to  actually  destroy  bacteria,  and  there  only  in  the 


SOIL   BACTEKIA,    CONDITIONS   FOR   GROWTH       401 

upper  surface.  In  fact,  it  is  seldom  that  soil  tempera- 
tures become  sufficiently  high  to  curtail  bacterial 
activity. 

272.  Organic    matter. — The   presence   of   a  certain 
quantity  of  organic  matter  is  essential  to  the  growth 
of  most,  but  not  all,  forms  of  soil  bacteria.    The  or- 
ganic matter  of  the  soil,  consisting  as  it  does  of  the 
remains  of  a  large  variety  of  substances,  furnishes  a 
suitable  food-supply  for  a  very  great  number  of  forms 
of  organisms.    The  action  of  one  set  of  bacteria  upon 
the   cellular   matter   of   plants   embodied   in   the   soil 
produces   compounds   suited   to   other  forms,    and   so 
from  one  stage  of  decomposition  to  another  this  con- 
stantly   changing    material    affords    sustenance    to    a 
bacterial  flora  the  extent  and  variety  of  which  it   is 
difficult  to  conceive.    Bacteria  not  only  affect  the  or- 
ganic matter  of  the  soil,  but,  in  the  case  of  certain 
forms,  their  activities  produce  changes  in  the  inorganic 
matter  that  cause  it  to  become  more  soluble  and  more 
easily  available  to  the  plant. 

A  soil  low  in  organic  matter  usually  has  a  lower 
bacterial  content  than  one  containing  a  larger  amount, 
and,  under  favorable  conditions,  the  beneficial  action, 
to  a  certain  point  at  least,  increases  with  the  content 
of  organic  substance;  but,  as  the  products  of  bacterial 
life  are  generally  injurious  to  the  organisms  producing 
them,  such  factors  as  the  rate  of  aeration  and  the 
basicity  of  the  soil  must  determine  the  effectiveness 
of  the  organic  matter. 

273.  Soil  acidity. — A  soil  having  an  acid  reaction 
makes  a  poor  medium  for  the  growth  of  bacteria.    A 


402         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 

neutral  or  slightly  alkaline  soil  furnishes  the  most 
favorable  conditions  for  bacterial  growth.  The  activi- 
ties of  many  soil  bacteria  result  in  the  formation  of 
acids  which  are  injurious  to  the  bacteria  themselves, 
and,  unless  there  is  present  some  basic  substance 
with  which  these  can  combine,  bacterial  development 
is  inhibited  by  their  own  products.  This  is  one  of  the 


Fio.   110.     Spring-toothed  walking  cultivator.    For  thorough,  shallow  tillage. 

reasons  why  lime  is  so  often  of  great  benefit  when  ap- 
plied to  soils,  and  especially  to  those  on  which  legumi- 
nous crops  are  growing.  For  the  same  reason,  the 
presence  of  lime  hastens  decay  of  organic  matter  in 
certain  soils,  and  the  conversion  of  nitrogenous  ma- 
terial with  a  minimum  loss  into  compounds  available 
to  the  plant.  As  showing  the  value  of  lime  in  the 
process  of  nitrification,  it  has  been  pointed  out  that 
in  the  presence. of  an  adequate  supply  of  lime  the 
availability  of  ammonium  salts  is  almost  as  high  us 


FUNCTIONS   OF  SOIL    BACTERIA  403 

that  of  nitrate  salts,  but  where  the  supply  is  insufficient 
the  value  of  ammonium  salts  is  relatively  quite  low. 

274.  Functions    of    soil    bacteria. — Bacteria    have 
a   part   in   many   of  the   processes  of  the  soil   which 
greatly    affects    its    productiveness.      It    has    become 
customary  to  refer  to  the  changes  produced  by  certain 
forms   of   bacteria   as   their   function   in    contributing 
to   soil-productiveness. 

275.  Decomposition    of    mineral    matter. — Certain 
bacteria   decompose   some   of   the   mineral    matter   of 
the  soil   and   render  it   more  easily   available  to  the 
plant.     While  the  nature  of  the  processes  and  their 
extent  are  not  known,  there  is  sufficient  evidence  to 
justify  the  above  statement.     It   is  well   known  that 
several  forms  of  bacteria  are  instrumental  in  decom- 
posing rock,  and  that  sulfur  and  iron  compounds  are 
acted  upon  by  other  forms.    Again,  the  much  greater 
efficiency   of   difficultly   soluble    phosphate   fertilizers, 
when  used  in  conjunction  with  a  quantity  of  organic 
matter,  is  evidence  of  the  relation  of  bacterial  action 
to  the  decomposition  of  mineral  substances.    Stocklasa 
has  shown  that,  when    B.  megatherium    and    B.  fluor- 
escent   are    added     to    soil    fertilized    with    insoluble 
phosphates,    plants   grown   thereon    take   up   a   larger 
amount    of    phosphorus    than    those   on    uninoculated 
soils. 

Organic  acids  and  carbon  dioxid  are  constantly 
produced  by  soil  bacteria.  These  in  soil  water  are 
weak  but  ever-acting  solvents,  the  effect  of  which 
must  in  the  end  be  considerable.  It  seems  likely, 
however,  that  there  is  a  more  direct  effect  of  certain 


404          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

bacteria  upon  mineral  matter  than  merely  the  solvent 
action  of  these  acids.  That  rock  may  be  disintegrated 
through  the  action  of  bacteria  has  been  already  com- 
mented upon.  Although  it  has  not  yet  been  demon- 
strated, bacteria  such  as  are  capable  of  decompos- 
ing rock  may,  in  all  probability,  exist  in  the  soil 
where  their  activities  result  in  the  "weathering"  that 
always  goes  on  in  soils  even  when  no  organic  mat- 
ter is  present. 

It  has  been  suggested  that  carbon  dioxid  dissolved 
in  water  may  act  on  the  very  difficultly  soluble  tri- 
calcium  phosphate,  producing  di-calcium  phosphate, 
a  more  soluble  form,  and  calcium  bicarbonate,  thus: 

Ca3(P04),  +  2CO,  +  2H20=Ca3H2(P04)a  +  Ca(HCO3)  , 

The  calcium  bicarbonate  thus  produced,  as  well 
as  that  derived  from  other  sources,  may  then  act  on 
the  double  silicates  of  aluminum  and  one  of  the  alkalies, 
thus: 


K,O.  A1A-  6  SiOa  +  Ca(HCO3),  =  CaO.  A13O3-  6  SiO2  +  2  KHCO, 

There  is  then  another  nutrient  rendered  available  to 
the  plant. 

It  has  been  shown  by  Van  Delden  and  by  Nadson 
that  several  forms  (M.  desulfuricans,  M.  cestuarii, 
Proteus  vulgaris  and  B.  mycoides)  are  able  to  reduce 
sulfates,  while  transformations  of  iron,  silicon  and 
calcium  are  effected  by  Proteus  vulgaris. 

276.  Decomposition  of  non-nitrogenous  organic 
matter.  —  The  organic  matter  commonly  decomposed 
in  soils  contains  a  large  proportion  of  compounds 


FUNCTIONS   OF   SOIL    BACTERIA  405 

containing  no  nitrogen.  The  non-nitrogenous  sub- 
stances decompose  quite  rapidly,  and  the  organic 
nitrogen  disappears  less  rapidly  than  the  carbon, 
hydrogen  and  oxygen  of  organic  bodies. 

Humus  always  contains  a  higher  percentage  of 
nitrogen  than  do  the  plants  from  which  it  is  formed 
(page  123). 

The  non-nitrogenous  substances  consist  of  cellulose 
and  allied  compounds  forming  the  cell-walls  of  plants, 
and  the  carbohydrates,  organic  acids,  fats,  etc.,  con- 
tained in  them.  The  dissolution  of  cellulose  is  brought 
about  by  the  action  of  the  enzyme  cytase  secreted 
by  a  number  of  fungi,  and  is  also  probably  accomplished 
by  the  Bacillus  amijlobacter,  but  whether  through  the 
secretion  of  an  enzyme  is  not  known.  Other  bacteria 
have  been  reported  to  secrete  a  cytase  that  acts  on 
certain  constituents  of  the  cell-wall.  It  is  probable 
that  numerous  organisms  capable  of  fermenting  cellu- 
lose and  allied  substances  exist  in  the  soil,  which 
decomposition  they  accomplished  through  the  pro- 
duction of  cytase. 

The  effect  of  cytase  upon  cellulose  and  other  fiber 
is  to  hydrolyse  it  with  the  formation  of  sugar,  as  glu- 
cose, mannose,  zylose.  aribinose.  etc. 

Starch  is  converted  into  glucose  by  a  ferment 
(diastase)  either  present  in  the  plant  itself  or  possibly 
secreted  by  fungi  or  bacteria.  All  the  sugars  are  finally 
converted  into  organic  acids  which  may  combine  with 
mineral  bases.  Distinct  organisms  have  boon  isolated 
that  can  utilize  for  their  development  formates,  acetates 
propionates,  butyrates,  etc..  the  final  product  being 


406         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

carbon  dioxid  and  water.  Thus,  step  by  step,  the  non- 
nitrogenous  matter  incorporated  in  the  soil  is  carried 
by  one  and  another  form  of  organisms  from  the  most 
complex  to  the  simplest  combinations. 

The  final  product  of  the  decomposition  of  carbon- 
aceous matter  being  carbon  dioxid,  there  is  a  return 
to  the  air  of  the  compound  from  which  the  carbon  of 
the  decomposing  substance  was  originally  derived. 
In  the  plant,  unless  it  is  saprophytic,  the  carbon  of 
the  tissues  comes  directly  from  the  carbon  dioxid  of 
the  air,  from  which  more  complex  carbon-bearing 
compounds  are  produced  and  utilized  in  its  functions 
or  in  its  tissues.  A  portion  of  the  carbon  is  returned 
to  the  air  by  the  plant  in  the  form  of  carbon  dioxid, 
the  remainder  is  retained  by  the  plant,  and  may  be 
returned  by  the  process  of  decay,  or  may  be  consumed 
by  an  animal,  and,  as  the  result  of  its  physiological 
processes,  either  exhaled  as  carbon  dioxid  or  deposited 
in  the  tissues  to  be  later  decomposed  and  converted 
into  carbon  dioxid.  The  soil  is  thus  the  scene  of  at 
least  a  part  of  the  varied  transformations  through 
which  carbon  is  continually  passing,  as  it  is  utilized 
by  higher  plants,  animals,  bacteria  and  fungi. 

The  non-nitrogenous  organic  substances  in  their 
various  stages  furnish  food  for  a  large  number  of 
bacteria,  among  which  are  those  concerned  in  the 
decomposition  of  mineral  matter  and  in  the  processes 
of  nitrification  and  nitrogen-fixation.  There  are,  there- 
fore, two  ways  in  which  these  substances  are  of  great 
importance  in  soil  fertility:  (1)  As  a  source  of  organic 
acids.  (2)  As  a  food-supply  for  useful  soil  bacteria. 


DECAY   BACTERIA  407 

277.  Decomposition  of  nitrogenous  organic  matter. 
—The  decomposition  of  nitrogenous  organic  matter  is 
accomplished  by  a  series  of  changes  from  one  compound 
to  another,  as  we  have  seen  was  the  case  with  the  non- 
nitrogenous  materials.  The  final  products  are  carbon 
dioxid,  water,  and  usually  some  hydrocarbon  gases 
resulting  from  the  carbon  and  hydrogen  of  the  organic 


Fio.  111.     The  large-shovel  riding  cultivator. 

matter,  and  also  some  hydrogen  sulfide  or  other  gas 
containing  sulfur  or  a  final  oxidation  of  the  sulfur  of 
the  proteids  into  sulfates,  while  the  nitrogen  is  ulti- 
mately converted  into  nitrates,  or  into  free  nitrogen, 
although  a  portion  of  the  original  nitrogen  some- 
times escapes  into  the  air  in  the  intermediate  stage, 
ammonia. 

The  processes  will  be  discussed  under  the  following 
heads,  which  represent  certain  more  or  less  definite 
stages  in  the  decomposition:  (1)  Decay  and  put  re- 


408         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

faction.  (2)  Ammonification.  (3)  Nitrification.  (4) 
Denitrification. 

278.  Decay  and  putrefaction.  —  Decomposition  of 
the  nitrogenous  organic  matter  of  the  soil,  consisting 
largely  of  the  proteins,  begins  with  either  one  of  two 
processes  —  decay  or  putrefaction.  Decay  is  produced 
by  aerobic  bacteria,  and  naturally  occurs  when  the 
conditions  are  most  favorable  for  their  development. 
When  the  conditions  are  otherwise,  the  growth  of 
these  bacteria  is  checked,  and  then  further  decom- 
position would  be  extremely  slow  were  it  not  for  the 
other  process  —  putrefaction.  Putrefaction  is  produced 
by  anaerobic  bacteria.  In  the  same  body,  and  conse- 
quently in  the  same  soil,  decay  and  putrefaction  may 
be  in  progress  simultaneously,  decay  taking  place  on 
the  outside  and  on  the  surfaces  of  other  parts  exposed 
to  the  air,  while  putrefaction  occurs  on  the  interior, 
where  the  supply  of  oxygen  is  limited.  By  means  of 
the  two  processes,  decomposition  is  greatly  facilitated. 

Decay  produces  a  very  rapid  and  complete  decom- 
position of  the  substance  in  which  it  operates,  most  of 
the  carbon  and  hydrogen  being  quickly  converted  into 
carbon  dioxid  and  water,  and  the  nitrogen  into  am- 
monia and  probably  some  free  nitrogen.  The  latter 
is  possibly  due  to  the  oxidation  of  ammonia,  thus 


The  sulfur  of  the  proteins  finally  appears  in  the  form 
of  sulfates. 

What  the  intermediate  products  are  has  not  been 
determined,  but  in  the  decay  of  meat,  where  there  was 


PUTREFACTION    BACTERIA  409 

an  abundant  supply  of  oxygen,  succinic,  palmytic, 
oleic  and  phenyl-propionic  acids  have  been  found. 

Putrefaction  results  in  a  large  number  of  complex 
intermediate  compounds  and  proceeds  much  more 
slowly.  Many  of  the  substances  thus  produced  are  highly 
poisonous  and  most  of  them  have  a  very  offensive 
odor.  They  may  be  further  broken  down  by  decay  when 
the  conditions  are  suitable,  or  by  a  continuation  of  the 
process  of  putrefaction.  In  either  case,  the  poisonous 
properties  and  the  odor  are  removed. 

In  the  process  of  decomposition  of  organic  matter 
two  classes  of  substances  are  produced:  (1)  Those 
which  have  been  excreted  or  secreted  by  the  bacterium, 
and  therefore  have  passed  through  the  metabolic 
processes  of  the  organism.  (2)  Those  that  have  been 
formed  because  of  the  removal  of  certain  atoms  by 
bacteria  or  enzymes  from  compounds,  thus  necessi- 
tating a  readjustment  of  the  remaining  atoms  and  the 
consequent  formation  of  a  new  compound. 

Putrefaction  is  carried  on  by  a  large  number  of 
forms  of  bacteria,  the  resulting  product  depending  upon 
the  substance  in  process  of  decomposition,  and  upon 
the  bacteria  involved.  Some  of  the  characteristic, 
although  not  constant  products,  formed  in  the  putre- 
faction of  albumin  and  proteins  are  albumenoses,  pop- 
tones,  and  amino-acids,  followed  by  the  formation 
of  cadaverin,  putrescin,  skatol  and  indol.  Where  an 
abundant  supply  of  oxygen  is  present,  or  where  a 
sufficient  supply  of  carbohydrates  exist,  these  sub- 
stances are  not  formed.  There  are  many  other  products 
of  putrefaction,  including  a  number  of  gases,  as  carbon 


410         THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

dioxid,  hydrogen  sulfide,  marsh  gas,  phosphine,  hy- 
drogen, nitrogen,  etc. 

It  will  be  noticed  that  these  changes,  like  those 
occurring  in  the  non-nitrogenous  organic  matter, 
involve  a  breaking  down  of  the  more  complex  com- 
pounds and  the  formation  of  simpler  ones;  that  a  very 
large  number  of  bacteria  are  concerned  in  the  various 
steps,  while  even  the  same  substances  may  be  decom- 
posed and  the  same  resulting  compounds  formed 
by  a  number  of  different  species  of  bacteria. 

Present-day  knowledge  of  the  subject  does  not 
make  it  possible  to  present  a  list  of  the  bacteria  con- 
cerned in  each  step,  or  to  name  all  of  the  intermediate 
products  formed;  but  for  the  student  of  the  soil  the 
principal  consideration  is  a  knowledge  of  the  circum- 
stances under  which  the  nitrogen  is  made  available  to 
plants,  and  the  conditions  which  are  likely  to  result 
in  its  loss  from  the  soil. 

279.  Ammonification. — Decay  and  putrefaction 
may  be  considered  as  a  continuation  of  ammonification, 
or  the  latter  process  as  the  beginning  of  the  former. 
Ammonification,  as  its  name  implies,  is  that  stage  of 
the  process  during  which  ammonia  is  formed  from  the 
intermediate  products. 

Like  the  other  processes  of  decomposition,  there 
are  many  species  of  bacteria  capable  of  forming  am- 
monia from  nitrogenous  organic  substances.  Differ- 
ent forms  display  different  abilities  in  converting  nitro- 
gen of  the  same  organic  material  into  ammonia,  some 
acting  more  rapidly  or  more  thoroughly  than  others. 
In  tests  by  certain  investigators  where  the  same  bac- 


A  MM  ON  I  PICA  TION  411 

teria  are  used  upon  different  substances,  the  order  of 
their  efficiency  is  changed  with  the  change  of  sub- 
stance. It  seems  likely,  therefore,  that  certain  forms 
are  most  efficient  when  acting  on  certain  organic  com- 
pounds. That,  in  other  words,  each  species  is  best 
adapted  to  the  decomposition  of  certain  substances, 
while  capable  of  attacking  others,  although  less  effec- 
tively. 

Among  the  bacteria  producing  ammonification 
are  B.  mycoides,  B.  subtilis,  B.  mesentericns  vulgatux, 
B.  janthinus  and  Proteus  vulgaris.  Of  these,  B.  mycoidcs 
has  been  very  carefully  studied,  and  the  findings  of 
Marchal  may  be  taken  as  representative  of  the  process 
of  ammonification.  He  found  that  when  this  bacterium 
was  seeded  on  a  neutral  solution  of  albumin,  ammonia 
and  carbon  dioxid  were  produced,  together  with  small 
amounts  of  peptones,  leuoin,  tyrosin,  and  formic, 
butyric  and  proprionic  acids.  He  concludes  that  in 
the  process,  atmospheric  oxygen  is  used,  and  that 
the  carbon  of  the  albumin  is  converted  into  carbon 
dioxid,  the  sulfur  into  sulfuric  acid,  the  hydrogen 
partly  into  water,  and  partly  into  ammonia  by  com- 
bining with  the  nitrogen  of  the  organic  substance. 
He  suggests  that  a  complete  decomposition  of  the  al- 
bumin occurs  according  to  the  following  reaction: 

C^H.^N^SO,,  +  770,  =  29  H,O -I- 72( '(),  +  S( ), 4- 1 8X H ,. 

The  greatest  activity  occurred  at  a  temperature  of 
86°  Fahr.,  and  as  low  as  6S°  Fahr.  action  was  quite 
strong.  Access  of  an  increased  amount  of  air,  produced 
by  increasing  the  surface  of  the  liquid,  increased  the 


412          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

rate  of  amraonification.  A  slightly  acid  reaction  in  the 
liquid  produced  the  maximum  activity,  but  in  a  neu- 
tral or  even  slightly  acid  medium  the  process  was 
continued,  although  much  less  actively. 

He  found  that  B.  mycoides  was  also  capable  of 
ammonifying  casein,  fibrin,  legumin,  glutin,  myosin, 
serin,  peptones,  creatin,  leucin,  tyrosin  and  asparagin, 
but  not  urea  and  ammonium  salts. 

280.  Nitrification. — Some  agricultural  plants  can 
utilize  ammonium  salts  as  a  source  of  nitrogen.  This 
has  been  determined  for  maize,  oats,  barley  and  po- 
tatoes. Other  plants,  such  as  beets,  show  a  decided 
preference  for  nitrogen  in  the  form  of  nitrates.  Whether 
any  of  the  common  crops  can  thrive  as  well  on  ammo- 
nium salts  as  upon  nitrates  has  not  been  finally  demon- 
strated. In  all  arable  soils  the  transformation  of 
nitrogen  does  not  stop  with  its  conversion  into  am- 
monia, but  proceeds  by  an  oxidation  process  to  the 
formation  of  first  nitrous  and  then  nitric  acids.  This 
may  be  considered  to  proceed  according  to  the  fol- 
lowing equations: 

2NH,+3O,=2HNOa+2HaO. 
2HNOS+O,  =  2HN03. 

The  acid  in  either  case  combines  with  one  of  the  bases 
of  the  soil,  usually  calcium,  so  that  we  have  calcium 
nitrate  resulting. 

Each  of  these  steps  is  brought  about  by  a  distinct 
bacterium,  but  they  are  closely  related.  Collectively 
they  are  called  nitro-bacteria.  Nitrosomonas  and 
Nitrosococcus  are  the  bacteria  concerned  in  the 


NITRIFICATION  413 

conversion  of  ammonia  into  nitrous  acid  or  nitrites. 
The  former  are  supposed  to  be  characteristic  of  Euro- 
pean, and  the  latter  of  American  soils.  They  are 
sometimes  referred  to  as  nitrous  ferments. 

Nitrobacter  are  those  bacteria  that  convert  nitrites 
into  nitrates.  They  are  also  designated  nitric  ferments. 
There  seem  to  be  some  differences  in  bacteria  from 
different  soils,  but  the  differences  are  slight,  and  the 
conditions  favoring  their  actions  are  similar.  It  is 
also  true  that  the  conditions  favoring  the  action  of 
Nitrosomonas  and  Nitrobacter  are  similar,  and  they 
are  generally  found  in  the  same  soils,  although  some 
experiments  show  that,  in  the  same  soil,  nitrites 
may  sometimes  accumulate,  indicating  conditions 
more  favorable  to  the  development  of  the  Nitrosomonas 
bacteria.  The  formation  of  nitrates  usually  follows 
closely  on  the  production  of  nitrites,  so  that  there  is 
rarely  more  than  a  trace  of  the  latter  to  be  found  in 
soils.  A  soil  favorable  to  the  process  of  nitrification 
is  usually  well  adapted  to  all  of  the  processes  of  nitro- 
gen transformation. 

Marked  differences  have  been  found  in  the  nitri- 
fying power  of  bacteria  from  different  soils.  Highly 
productive  soils  have  generally  been  found  to  contain 
bacteria  having  greater  nitrifying  efficiency  than  those 
from  less  productive  soils,  but  this  may  not  always 
be  the  case,  as  other  factors  may  limit  the  productive- 
ness. 

281.  Effect  of  organic  matter  on  nitrification. — 
A  peculiarity  in  the  artificial  culture  of  nitrifying 
bacteria  is  that  they  cannot  be  grown  in  artificial 


414 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


medium  containing  organic  matter.  This  property 
for  a  long  time  prevented  the  isolation  and  identifi- 
cation of  these  organisms,  as  it  was  hardly  conceivable 
that  organisms  living  in  the  dark,  where  energy  can- 
not be  obtained  from  sunlight,  could  exist  without 
using  the  energy  stored  by  organic  matter.  It  has 
been  suggested,  in  explanation  of  this,  that  the  energy 


FIG.  112.  Curves  showing  the  relation  of  the  moisture  and  temp>erature  of 
the  soil  to  the  formation  of  nitrates  which  are  given  in  parts  per  million  of  dry 
soil.  Depth  of  sampling,  eight  inches.  These  curves  bring  out  clearly  the  fact 
that  the  warmer  soil  temperature,  combined  with  a  moderately  high  soil  mois- 
ture content  favors  the  formation  of  nitrates. 

produced  by  the  oxidation  involved  in  the  process 
of  nitrification  makes  possible  the  growth  of  the  or- 
ganisms under  these,  apparently  impossible,  condi- 
tions. Some  experimenters  report  having  grown 
nitrobacteria  in  organic  media,  but  it  is  generally 
believed,  at  present,  that  this  is  not  possible,  and  that 
there  has  been  some  error  in  their  work. 

The  presence  of  peptone  in  the  proportion  .of  500 
parts   per    million   completely    prevents   the   develop- 


NITRIFICATION  415 

ment  of  nitrobacteria  and  one  half  that  quantity  checks 
it,  while  150  parts  of  ammonia  per  million  has  a  similar 
effect.  In  a  normal  soil,  the  quantity  of  soluble  am- 
monium salts  is  well  below  this  amount,  as  must  also 
be  that  of  soluble  organic  matter.  In  confirmation  of 
the  inhibiting  effect  of  organic  matter  on  the  nitrobac- 
teria, cases  have  been  reported  of  soils  very  rich  in 
organic  matter  in  which  no  bacteria  of  this  type  occur. 

It  has  also  been  stated  that  very  heavy  manuring 
with  organic  manures  results  in  decreased  nitrification 
in  the  soil.  While  this  may  be  true  where  farm  manure 
is  used  in  the  quantities  sometimes  applied  in  garden- 
ing operations,  it  is  not  likely  to  occur  in  soils  on  which 
ordinary  field  crops  are  grown.  The  principle  is  well 
illustrated  by  the  dry-earth  closet.  Manure  mixed  with 
earth  in  relatively  small  proportions  and  kept  aerated 
by  occasional  mixing  undergoes  a  very  thorough  decom- 
position of  the  manure  but  without  any  corresponding 
increase  in  nitrates.  On  the  other  hand,  under  field  con- 
ditions, manure  used  in  relatively  small  amounts  does 
not  undergo  this  serious  loss. 

The  application  of  twenty  tons  of  farm  manure 
per  acre  to  sod  on  a  clay  loam  soil  for  three  consecu- 
tive years,  at  Cornell  University,  resulted  in  a  larger 
production  of  nitrates  on  the  manured  soil  than  upon 
a  contiguous  plat  of  similar  soil  left  unmanured. 
This  was  true  during  the  third  year  of  the  applications, 
when  the  land  was  in  sod,  and  also  the  fourth  year 
when  no  manure  was  applied  to  either  plat,  and  when 
both  were  planted  to  corn,  as  may  be  seen  from  the 
following  table: 


416 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


TABLE  LXII. — NITRATES  PRODUCED  ON  HEAVILY  MANURED  AND 
ON  UNMANURED  SOIL 


NOs  in  parts  per  million,  dry  soil 

Unmanured  soil 

Twenty  tons  ma- 
nure per  acre  for 
three  years 

Land  in  timothy  —  . 
April  23  

8.2 
4.1 
3.3 
2.0 
2.4 
0.8 
1.3 
2.2 
1.8 

17.5 
42.8 
50.0 
195.0 
151.0 

21.0 
4.6 
4.5 
40 
2.0 
1.1 
3.0 
2.8 
3.0 

20.1 
79.3 
105.0 
304.0 
184.0 

May  3  

May  14  

May  30  

June  1  

June  13  

June  20  

July  24     

August  14  

Land  in  maize- 
May  19  :  

June  22  

July  6     

July  28             

August  10  ...            .          ... 

282.  Effect  of  soil  aeration  on  nitrification. — • 
Probably  the  most  potent  factor  governing  nitrifi- 
cation in  the  soil  is  the  supply  of  air.  In  clay  and  even 
in  loam  soils,  the  tendency  to  compactness  is  such 
as  to  exclude  air  sufficient  to  enable  nitrification  to 
proceed  as  rapidly  as  desirable  unless  the  soil  be  well 
tilled.  Columns  of  soil  eight  inches  in  diameter  and 
of  the  same  depth  were  removed  from  a  field  of  clay 
loam  on  Cornell  University  farm,  and  carried  to  the 
greenhouse  without  disturbing  the  structure  of  the 
soil  as  it  existed  in  the  field.  At  the  same  time,  simi- 
lar-sized vessels  were  filled  with  soil  dug  up  from  a 


SOIL   CONDITION   AND  NITRIFICATION 


417 


spot  nearby.  These  may  be  termed  unaerated  and 
aerated  soils.  Both  were  kept  at  the  same  temperature 
and  moisture  content  in  the  greenhouse,  but  no  plants 
were  grown  upon  them.  The  production  of  nitrates 
was  as  follows: 

TABLE  LXIII 


Date  of  analysis 


When  taken  from  field  .... 
After  standing  one  month  . 
After  standing  two  months 


Nitrates  in  dry  soil,  parts  per  million 


Unaerated  soil 


3.2 
4.2 
9.0 


Aerated  soil 


3.2 
17.6 
45.G 


283.  Effect  of  sod  on  nitrification.  —  Nitrification 
proceeds  slowly  on  sod  land,  especially  if  the  soil  is 
heavy.  On  the  same  type  of  soil  as  that  used  in  the 
experiment  last  described,  the  average  quantities  of 
nitrates  for  each  month  of  the  growing  season  in  the 
surface  eight  inches  of  sod  land  as  compared  with 
maize  land  under  the  same  manuring  were  as  follows: 

TABLK  LXIV 


Nitrates  in  dry  soil,  part*  p«>r  million 


Month 

Sod  land 

Maiie  land 

April          

8.9 

May  

3.0 

17.1 

June   

2.4 

40.3 

July  

4.0 

194.0 

August  

5.4 

18(3.7 

A  A 


418          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

The  amount  of  nitrogen  removed  by  the  maize 
crop  was  greater  than  that  removed  by  the  timothy, 
consequently  the  greater  amount  in  the  former  soil 
can  not  be  due  to  the  effect  of  the  crop. 

So  far  as  the  conservation  of  nitrogen  is  concerned, 
sod  is  an  ideal  crop,  for  nitrates  are  formed  very  little 
faster  than  they  are  used,  and  are  not  carried  off  in 
large  amounts  by  the  drainage  water. 

In  the  corn  land  as  much  as  500  pounds  of  nitrates 
were  present  in  the  first  twelve  inches  of  one  acre,  or 
fully  five  times  as  much  as  was  used  by  the  crop. 

284.  Depth  at  which  nitrification  takes  place.— 
Warington  concluded  from  his  experiments  that 
nitrification  takes  place  only  in  the  surface  six  feet  of 
soil.  Hall  has  pointed  to  the  fact  that  no  more  nitrates 
were  leached  from  the  60-inch  lysimeter  at  Rotham- 
sted  than  from  the  one  20  inches  deep;  which  is  very 
good  evidence  that  in  that  particular  soil  nitrification 
does  not  take  place  below  20  inches  from  the  surface. 
In  more  porous  soils,  however,  nitrification  probably 
extends  deeper,  especially  in  the  rich  and  porous 
subsoils  of  the  arid  and  semi-arid  regions. 

In  all  probability,  nitrification  is  largely  confined 
to  the  furrow  slice,  where  the  opening  up  of  the  soil 
by  tillage  has  provided  the  necessary  air,  and  where  the 
temperature  rises  to  a  point  more  favorable  to  the  action 
of  nitrifying  bacteria.  The  results  from  the  aerated 
and  unaerated  soils  cited  above  represent  the  differ- 
ences that  doubtless  exist  between  the  furrow  slice  and 
the  subsoil  so  far  as  nitrification  is  concerned. 

286.  Loss  of  nitrates  from  the  soil. — Nitrogen  hav- 


SOIL   CONDITION   AND   NITRIFICATION 


419 


ing  been  converted  into  the  form  of  nitric  acid,  it  im- 
mediately combines  with  available  bases  in  the  soil 
forming  salts,  all  of  which  are  very  easily  soluble,  and 
which  are  carried  in  solution  by  the  soil  water.  In  a 
region  of  large  rainfall,  the  removal  of  nitrates  in  the 
drainage  water  is  very  rapid.  Hall  states  that  nitrates 
formed  during  the  summer  or  autumn  of  one  year  are 
practically  all  removed  from  the  soil  of  the  Rot  ha  m- 


Fio.  113.    The  modern,  small,  eight-shovel  riding  cultivator. 


sted  fields  before  the  crops  of  the  following  year  have 
advanced  sufficiently  to  utilize  them.  It  was  formerly 
customary  to  fertilize  with  ammonium  salts  in  the 
autumn,  but  the  drainage  water  showed  on  analysis 
such  a  large  quantity  of  nitrates  during  the  months 
intervening  between  the  time  of  fertilizing  and  the 
opening  of  the  growing  season  that  the  practice  was 
discontinued. 

In    regions    of    less    rainfall    or    of    urrator    surface 
evaporation,   the  loss   in   this   way   is   less,   reaching  a 


420          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

minimum  in  an  arid  region  when  irrigation  is  not  prac- 
ticed. Under  such  conditions,  there  is  a  return  of  ni- 
trates to  the  upper  soil,  as  capillary  water  moves 
upward  to  replace  evaporated  water.  In  fact,  wherever 
evaporation  takes  place  to  any  considerable  extent, 
there  is  some  movement  of  this  kind.  The  need  for 
catch  crops  to  take  up  and  preserve  nitrogen  is  there- 
fore greater  in  a  humid  region  than  in  an  arid  or  semi- 
arid  one.  An  arrangement  of  crops  that  allows  the 
land  to  stand  idle  for  some  time,  or  a  crop  that  requires 
intertillage,  as  does  maize,  fails  to  utilize  all  of  the 
nitrates  produced,  and  promotes  the  loss  of  nitrogen 
in  drainage  water. 

286.  Denitrification. — The  nitrogen  transforming 
bacteria  thus  far  studied  have  been  those  that  cause 
the  oxidation  of  nitrogen  as  the  result  of  their  activi- 
ties. We  may  now  consider  a  number  of  forms  of  bac- 
teria that  accomplish  a  reverse  action.  The  several 
processes  involved  are  commonly  designated  by  the 
term  denitrification,  and  comprise  the  following: 

(1)  Reduction   of   nitrates   to   nitrites   and   ammonia. 

(2)  Reduction    of   nitrates   to    nitrites,    and   these   to 
elementary  nitrogen. 

The  number  of  organisms  that  possess  the  ability 
to  accomplish  one  or  more  of  these  processes  is  very 
large, — in  fact  greater  than  the  number  involved  in 
the  oxidation  processes, — but,  in  spite  of  their  numbers, 
permanent  loss  of  nitrogen  in  ordinary  arable  soils  is 
unimportant  in  amount,  although  in  heaps  of  barnyard 
manure  it  may  be  a  very  serious  cause  of  loss. 

Some  of  the  specific  bacteria  reported  to  bring  about 


DENITRIFICA  TION  421 

denitrification  are:  B.  ramosus  and  B.  pcstifer,  which 
reduce  nitrates  to  nitrites;  B.  mijcoidcK,  B.  subtilis,  B. 
mesentericus  vulgatus  and  many  other  ammonification 
bacteria  which  are  capable  of  converting  nitrates  into 
ammonia. 

Bacterium  dcnitrificans  alpha  and  Bacterium  dcni- 
trificans  beta  reduce  nitrates  with  the  evolution  of 
gaseous  nitrogen. 

In  addition  to  these  nitrate-destroying  bacteria, 
there  arc  other  bacteria  which  also  utilize  nitrates; 
but,  like  higher  plants,  they  convert  (he  nitrogen  into 
organic  nitrogenous  substances.  However,  as  they 
operate  in  the  dark  and  cannot  obtain  energy  from 
sunlight,  they  must  have  organic  acids  or  carbohy- 
drates as  a  source  of  energy.  While  those  bacteria 
cannot  be  considered  to  be  denitrifiers.  they  help  to 
deplete  the  supply  of  nitrates  when  conditions  are 
favorable  for  their  development.  What  these  condi- 
tions are  is  not  well  understood,  nor  can  any  estimate 
be  made  as  to  the  extent  of  their  operations. 

Most  of  the  nitrifying  bacteria  perform  their  func- 
tions only  under  a  limited  access  of  oxygen,  while 
others  can  operate  in  the  presence  of  a  more  liberal 
supply;  but,  in  general,  thorough  aeration  of  the  soil 
practically  prevents  denitrification.  Straw  and  dung 
apparently  carry  an  abundant  supply  of  denitrifying 
organisms,  and  also  furnish  a  supply  of  carbohydrates 
which  favors  their  action,  so  that  stable  manure  is 
very  likely  to  undergo  denitrification,  and  straw  or 
coarse  stable  manure  are  conducive  to  the  growth  of 
denitrifying  bacteria  in  the  soil. 


422         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

Under  ordinary  farm  conditions,  denitrification  is 
of  no  significance  in  the  soil  where  proper  drainage 
and  good  tillage  are  practiced.  Warington  showed  that, 
if  an  arable  soil  be  kept  saturated  with  water  to  the 
exclusion  of  air,  nitrates  added  to  the  soil  are  decom- 
posed, with  the  evolution  of  nitrogen  gas.  As  lack  of 
drainage  is  usually  most  pronounced  in  the  early 
spring,  when  the  soil  is  likely  to  be  depleted  of  nitrates, 
it  is  not  likely  that  much  loss  arises  in  this  way  unless 
a  nitrate  fertilizer  has  been  added.  Of  the  many  diffi- 
culties arising  from  poor  drainage,  denitrification  of 
an  expensive  fertilizer  may  be  very  considerable  item. 

The  addition  of  a  nitrate  fertilizer  to  a  soil  receiving 
stable  manure  is  not  likely  to  result  in  a  loss  of  ni- 
trates unless  the  dressings  of  manure  have  been  ex- 
tremely heavy.  Hall  states  that  at  Rothamsted,  where 
large  quantities  of  nitrate  of  soda  are  used  every 
year  in  connection  with  annual  dressings  of  farm 
manure,  the  nitrate  produces  nearly  as  large  an  in- 
crease when  added  to  the  manured  as  when  added 
to  the  unmanured  plat.  There  appears,  in  other  words, 
to  be  no  loss  of  nitrate  by  denitrification. 

It  is  possible  to  reach  a  point  in  manuring  where 
denitrification  may  take  place.  Market  gardeners 
sometimes  reach  this  point  where  fifty  tons  or  more 
of  farm  manure,  in  addition  to  a  nitrate  fertilizer, 
are  added  to  the  soil.  Plowing  under  heavy  crops 
of  green  manure  may  produce  the  same  result.  In 
either  case,  the  best  way  to  overcome  the  difficulty  is 
to  allow  the  organic  matter  to  partly  decompose 
before  adding  the  fertilizer.  The  removal  of  the  easily 


NITROGEN   FIXATION    BY    BACTERIA  423 

decomposable   carbohydrates   needed   by   the   denitri- 
fying organisms  decreases  or  precludes  their  activity. 

287.  Nitrogen     fixation     through     symbiosis     with 
higher  plants. — It  has  long  been  recognized  by  farmers 
that  certain  crops  like  clover,  alfalfa,  peas,  beans,  etc., 
improve  the  soil,   making  it   possible  to  grow  larger 
crops  of  cereals  after  these  crops  have  been  upon  the 
land.    The  benefit  was,  within  the  past  century,  traced 
to  an  increase  in  the  nitrogen  content  of  the  soil,  and 
the  specific  plants  so  affecting  the  soil  were  found  to  be, 
with  perhaps  a  few  exceptions,  those  belonging  to  the 
family  of  legumes.     It  has  furthermore  been  demon- 
strated that  these  plants  utilize,   under  certain   con- 
ditions, the  uncombined  nitrogen  of  the  atmosphere, 
and   that   they   contain,    both   in   the   aerial    portions 
and  in  the  roots,  a  very  high  percentage  of  nitrogen. 
In  consequence,  the  decomposition  of  even  the  roots 
of  the   plants   in   the   soil   leaves   a   large   amount   of 
nitrogenous  matter. 

288.  Relation   of   bacteria   to   nodules   on    roots. — 
It  has  also  been  shown  that  the  utilization  of  atmos- 
pheric   nitrogen   is   accomplished   through   the   aid   of 
certain    bacteria   that    live   in    nodules    (tubercles)    on 
the  roots  of  the   plants.    These  bacteria  acquire  tho 
free  nitrogen  from  the  air  in  the  soil,  and  the  host  plant 
secures   it    in   some  form   from   the   bacteria  or   thoir 
products.    The  presence  of  a  certain  species  of  bacteria 
is  necessary  for  the  formation  of  tubercles.     Legumi- 
nous plants  grown  in  cultures  or  in  soil  not  containing 
the  necessary  bacteria  do  not  form  nodules,   and  do 
not  utilize  atmospheric  nitrogen,  the  result  being  that 


424 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


the  crop  produced  is  less  in  amount  and  the  percentage 
of  nitrogen  in  the  crop  is  less. 

It  has  for  some  years  been  the  belief  that  the  or- 
ganism which  produces  the  nodules  and  utilizes  the 
uncombined  nitrogen  is  the  Pseudomonas  radicicola, 

but  this  has  very  lately 
been  called  in  ques- 
tion. 

The  nodules  are  not 
normally  a  part  of 
leguminous  plants  but 
are  evidently  caused 
by  some  irritation  of 
the  root  surface,  much 
as  a  gall  is  caused  to 
develop  on  a  leaf  or 
branch  of  a  tree  by  an 
insect.  In  a  culture 
containing  the  proper 
bacteria,  the  prick  of 
a  needle  on  the  root 
surface  will  cause  a 
nodule  to  form  in  the 
course  of  a  few  days. 
The  entrance  of  the 
bacteria  is  effected 
through  a  root -hair 
which  it  penetrates, 
and  may  be  seen  as 

Fio.  114.    Nodules  on    the   roots   of    an  r-i  •>• 

alfalfa  plant    Bacteria  live  in  these  nodules,  a     Filament     extending 

or  tubercles,  and  have  the  power  of  utilizing  j.u«         »'        1  *U  ^t  *U«. 

the  free  nitrogen  of  the  air  in  their  growth.  the  entire  length  OI  the 


TUBERCLE   BACTERIA  425 

hair,  and  into  the  cells  of  the  cortex  of  the  root, 
where  the  growth  of  the  tubercle  starts. 

Even  where  the  causative  bacteria  occur  in  cultures 
or  in  the  soil,  leguminous  plants  may  not  secure  any 
atmospheric  nitrogen,  or  perhaps  only  a  small  quantity, 
if  there  is  an  abundant  supply  of  readily  available 
combined  nitrogen  upon  which  the  plant  may  draw. 
The  bacteria  have  the  ability  to  utili/e  combined 
nitrogen  as  well  as  uncombined  nitrogen,  and  prefer  to 
have  it  in  the  former  condition.  On  soils  rich  in  nitro- 
gen legumes  may,  therefore,  add  little  or  no  nitrogen 
to  the  soil,  while  in  properly  inoculated  soils  deficient 
in  nitrogen  an  important  gain  of  nitrogen  results. 

While  /•*.  radicicola  has  been  considered  the  organism 
common  to  all  leguminous  plants,  it  is  now  known 
that  the  organisms  from  one  species  of  legume  are  not 
equally  well  adapted  to  the  production  of  tubercles 
on  each  of  the  other  species  of  legumes.  They  show 
greater  activity  on  some 'species  than  on  others,  but 
do  not  develop  so  successfully  on  any  species  as  on 
the  one  from  which  the  organisms  were  taken.  It  was 
quite  generally  believed  at  one  time  that  the  longer  any 
species  of  legume  is  in  contact  with  the  organisms 
from  another  species  the  more  active  they  become, 
and  the  greater  the  utilization  of  atmospheric  nitrogen. 
Considerable  doubt  has  been  cast  upon  this  view  in 
recent  years,  and  it  is  now  generally  conceded  that 
the  bacteria  of  certain  legumes  are  not  capable  of 
inoculating  certain  other  species  of  legumes. 

289.  Transfer  of  nitrogen  to  the  plant.  —  It  has 
been  shown  by  several  investigators  that  bacteria 


426          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

from  the  nodules  of  legumes  are  able  to  fix  atmos- 
pheric nitrogen  even  when  not  associated  with  legumi- 
nous plants.  There  would  seem  to  be  no  doubt,  there- 
fore, that  the  fixation  of  nitrogen  in  the  tubercles  of 
legumes  is  accomplished  directly  by  this  organism, 
and  not  by  the  plant  itself,  or  through  any  combina- 
tion of  the  plant  and  organism, — both  of  which  hy- 
potheses have  been  advanced.  The  part  which  the 
plant  plays  is  doubtless  to  furnish  the  carbohydrates 
required  in  large  quantities  by  all  nitrogen-fixing 
organisms  and  which  the  legumes  are  able  to  supply  in 
large  amounts.  The  utilization  of  large  quantities 
of  carbohydrates  by  the  nitrogen-fixing  bacteria  in 
the  tubercles  may  also  account  for  the  small  proportion 
of  non-nitrogenous  organic  matter  in  the  plants. 

Kow  the  plant  absorbs  this  nitrogen  after  it  has 
been  secured  by  the  bacteria  is  less  well  understood. 
Early  in  the  growth  of  the  tubercle,  a  mucilaginous 
substance  is  produced  which  permeates  the  tissues 
of  the  plant  in  the  form  of  long,  slender  threads,  and 
which  contain  the  bacteria.  These  threads  develop 
by  branching  or  budding,  and  form  what  have  been 
called  Y  and  T  forms  known  as  bacteroids,  which  are 
peculiar  to  these  bacteria,  and  not  produced  by  them 
when  grown  in  the  media  of  the  laboratory.  The  threads 
finally  disappear,  and  the  bacteria  diffuse  themselves 
more  or  less,  through  the  tissues  of  the  root.  What 
part  the  bacteroids  play  in  the  transfer  of  nitrogen  is 
not  known.  It  has  been  suggested  that  in  this  form 
the  nitrogen  is  absorbed  by  the  tissues  of  the  plant. 
It  seems  quite  likely  that  the  nitrogen  compounds 


SOIL-INOCULATION  427 

produced  within  the  bacteria  cells  are  diffused  through 
the  cell-wall  and  absorbed  by  the  plant. 

In  a  recent  report,  De  Rossi  states  that  Pseudo- 
monas  radicicola  is  not  the  causative  agent  in  the 
fixation  of  nitrogen  in  the  nodules  of  leguminous 
plants,  and  that  he  has  isolated  other  bacteria  that 
do  possess  this  property.  These  bacteria  produce  the 
Y  and  T  forms  in  artificial  media,  which  is  in  itself  an 
indication  of  their  identity  with  the  bacteria  concerned 
in  nitrogen-fixation.  De  Rossi's  work  may  also  explain 
why  what  was  formerly  considered  to  be  one  form 
of  bacterium,  Pscudomonas  radicicola,  common  to 
all  leguminous  plants,  is  not  capable  of  inoculating 
one  species  of  legume  when  transferred  from  another. 
It  may  be  that  there  are  a  number  of  different  forms, 
each  adapted  to  certain  species  of  legumes. 

290.  Soil-inoculation  for  legumes. — The  possibility 
of  securing  a  better  growth  of  leguminous  crops  on 
soils  not  having  previously  grown  such  a  crop  success- 
fully, was  conceived  immediately  following  the  dis- 
covery of  the  nitrogen-fixing  bacteria.  Extensive 
experiments  showed  the  practicability  of  inoculating 
land  for  a  certain  leguminous  crop  by  spreading  upon 
its  surface  soil  from  a  field  on  which  the  same  crop  is 
successfully  growing.  It  is  manifestly  much  bettor 
to  apply  the  organisms  or  a  certain  species  of  legume 
from  a  field  having  grown  the  same  species  than  to 
attempt  to  use  organisms  from  another  species  of  le- 
gume. The  fact  that  soil-inoculation  by  means  of  soil 
from  other  fields  may  possibly  transmit  wood  seeds 
and  fungous  diseases,  and  also  necessitates  the  trans- 


428         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

portation  of  a  great  bulk  and  weight  of  material, 
has  led  to  numerous  efforts  to  inoculate  soil  by  means 
of  pure  cultures.  The  pure  culture  may  also  make  it 
possible  to  bring  to  the  soil  bacteria  of  greater  physio- 
logical efficiency  than  those  already  there. 

The  first  attempt  at  inoculation  by  pure  cultures 
was  made  in  Germany,  the  cultures  being  sold  under  the 
name  of  "Nitragin."  Careful  experiments  made  with 
this  material  previous  to  the  year  1900  did  not  show 
it  to  be  very  efficient;  but,  of  recent  years,  improve- 
ments in  the  method  of  manipulating  the  cultures 
have  resulted  in  much  greater  success.  In  "  Nitragin, " 
the  medium  used  for  growing  the  organisms  is  gelatin, 
and,  before  use,  this  was  formerly  dissolved  in  water; 
but  now  a  solution  of  greater  density  is  used  in  order 
to  prevent  a  change  of  osmotic  pressure,  which  may 
cause  plasmolysis  and  result  in  the  destruction  of  the 
bacteria. 

Within  recent  years,  a  number  of  cultures  for  soil- 
inoculation  have  been  offered  to  the  public.  The  first 
of  these  utilized  absorbent  cotton  to  transmit  the 
bacteria  in  a  dry  state  from  the  pure  cultures  in  the 
laboratory  to  the  user  of  the  culture,  who  was  to 
prepare  therefrom  another  culture  to  be  used  for 
inoculating  the  soil.  Careful  investigation  of  this 
method  showed  that  its  weakness  lay  in  drying  the 
cultures  on  the  absorbent  cotton  which  frequently 
resulted  in  the  death  of  the  organisms.  More  recently, 
liquid  cultures  have  been  placed  on  the  market  in  this 
country,  but  they  have  not  yet  been  sufficiently  well 
tested  to  prove  their  efficiency.  It  is  undoubtedly 


NON -SYMBIOTIC   NITROGEN    FIXATION  429 

only  a  question  of  time  until  a  successful  method  of 
inoculating  soil  from  artificial  cultures  will  be  found. 
In  the  meantime,  inoculation  by  means  of  infested 
soil  is  the  most  practical  method. 

291.  Nitrogen-fixation      without      symbiosis      with 
higher  plants. — If  a  soil  be  allowed  to  stand  idle,  either 
without  vegetation  or  in  grass,  it  will,  under  favorable 
moisture  conditions,  in  the  northern  states,  accumu- 
late  in   one  or   two  years   an   appreciable   amount   of 
nitrogen  not  present  at  the  beginning  of  the  period. 
At  the   Rothamsted    Ivxperiment   Station,   one  of  the 
fields  in   volunteer  plants,   consisting   mainly  of  grass 
without  legumes,  gained  in  the  course  of  twenty  years 
about  twenty-five  pounds  of  nitrogen  per  acre,  annually. 
According  to  Hall,  the  nitrogen  brought  down  by  rain 
would    account    for   about    five    pounds    per   acre    por 
annum,    and    dust,    bird-droppings,    etc.,    for    a    little 
more.     As   pointed   out    by    Lipman,    there    must    also 
have  been  a  greater  total  accretion  of  nitrogen  during 
the  twenty  years  than   appears  in  the  final  result,   as 
considerable    must    have    been    lost    through    removal 
of  nitrates  in  drainage  and  escape  of  nitrogen  in  the 
ordinary   processes  of  its   transformation. 

292.  Nitrogen-fixing      organisms,      hired      experi- 
ment lias  shown  that  certain  bacteria  have  the  ability 
to  utilize  atmospheric  nitrogen  and  to  leave  it   in  the 
soil    in    a    combined    form.       A    bacillus     Clout  rid  ium 
pasteurianum —  was  first   found    to  produce  this  result. 
Later,  a  commercial  culture  called  "  Alinit  "  was  placed 
on  the  market  in  (lermany.  which  culture  it  was  claimed 
contained     Bacterium    cllcnbachcn.tis,    with    which    the 


430         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

soil  was  to  be  inoculated,  and  that  a  large  fixation 
of  atmospheric  nitrogen  would  result.  A  number  of 
tests  of  this  material  failed  to  show  that  it  caused  any 
marked  fixation  of  atmospheric  nitrogen. 

A  number  of  other  nitrogen-fixing  organisms 
have  since  been  discovered.  There  are:  (1)  Several 
members  of  the  group  designated  Azotobacter,  which 
are  aerobic  bacteria,  and  which  some  investigators 
hold  to  be  capable  of  fixing  atmospheric  nitrogen 
when  grown  in  pure  cultures,  and  others  believe  to 
be  able  to  do  so,  at  least  in  large  amounts,  only  in 
the  presence  of  certain  other  organisms.  (2)  Mem- 
bers of  the  Granulobacter  group,  which  are  large 
spore-bearing  bacilli  of  anaerobic  habits.  (3)  B. 
radiobacter,  which  appear  to  be  closely  related  to  or 
identical  with  the  B.  radicicola  of  legume  tubercles. 
The  latter  has  been  shown  to  be  able  to  fix  atmospheric 
nitrogen  even  when  not  growing  in  symbiosis  with 
legumes. 

There  are  doubtless  many  other  nitrogen-fixing 
organisms  still  to  be  discovered. 

A  peculiarity  of  these  nitrogen-fixing  organisms 
is  their  use  of  carbohydrates,  which  they  decompose 
in  the  process  of  nitrogen-fixation.  They  secure  more 
atmospheric  nitrogen  when  in  a  nitrogen-free  medium. 
The  presence  of  soluble  lime  or  magnesium  salts,  es- 
pecially carbonates,  is  necessary  for  the  best  per- 
formance of  the  nitrogen-fixing  function,  as  is  also 
the  presence  of  a  somewhat  easily  soluble  form  of 
phosphorus.  They  are  exceedingly  sensitive  to  an  acid 
condition  of  the  soil. 


NITROGEN-FIXATION  IN  PURE   CULTURES        431 

293.  Mixed  cultures  of  nitrogen-fixing  organisms. — 
Mixed   cultures   of  the   various  organisms   mentioned 
fix  larger  amounts  of  nitrogen  than  do  the  pure  cultures 
of  any  one  of  them,  while  some  forms  are  incapable 
of  fixing  nitrogen  in  pure  cultures.    Certain  algse,  par- 
ticularly the  blue-green  algae,  aid  greatly  in  promoting 
growth    and    nitrogen-fixation    by    these    organisms. 
This  they   probably  do  by  producing  carbohydrates, 
which  are  used  by  the  bacteria  as  a  source  of  energy 
for  nitrogen-fixation,  the  bacteria  furnishing  the  algse 
with    nitrogenous    compounds.     To    what    extent    the 
relation  is  symbiotic  is  not  known  at  present,  but  it 
seems  probable  that  a  relation   may  exist   similar  to 
that    between    leguminous    plants    and    the    nitrogen- 
gathering  bacteria  in  their  nodules. 

294.  Nitrogen-fixation  and  denitrification  antagonis- 
tic.— Nitrogen-fixation  and  denitrification  are  reverse 
processes.    The  former  is,  for  most  bacteria,   favored 
by    an    abundant    air-supply    and    a   moderately   high 
temperature.    Thus,  at  75°  Fahr.,  fixation  was  rapid; 
at    59°    Fahr.,    it    was    decreased,    and    at    44°    Fahr, 
there  was  none.    Denitrification  is  favored  by  a  some- 
what limited  supply  of  oxygen. 

There  is  no  reason  to  believe  that  the  practical 
importance  of  nitrogen-fixation  without  legumes  is 
equal,  under  the  most  favorable  conditions,  to  that 
with  legumes.  A  further  knowledge  of  the  organisms 
effecting  fixation  and  of  their  habits  will  doubtless 
make  possible  a  greater  utilization  of  their  powers, 
to  supplement  the  use  of  legumes,  as  a  source  of  com- 
bined nitrogen  in  the  soil. 


E.    THE   SOIL    AIR 

I.     FACTORS     DETERMINING     VOLUME 

The  amount  of  air  that  soils  contain  varies  with 
different  soils,  and  in  any  one  soil  it  varies  with  cer- 
tain changes  to  which  it  is  subject  from  time  to  time. 
The  factors  affecting  the  volume  of  air  in  soils  are: 
(1)  The  texture.  (2)  The  structure.  (3)  The  organic 
matter.  (4)  The  moisture  content. 

295.  Texture. — The  size  of  the  soil  particles  affect 
the  air  capacity  of  the  soil  in  exactly  the  same  way 
as  it  does   the   pore-space    (see   page  92),    since    the 
two  are  identical.    A  fine-textured  soil  in  a  dry  condi- 
tion would,  therefore,   contain   as  large  a  volume  of 
air  as  a  coarse-textured  one,   provided  the  particles 
were  spherical  and  all  of  the  same  size. 

Under  the  conditions  actually  existing  in  the  field, 
those  soils  composed  of  small  particles  generally 
possess  the  larger  air-space. 

296.  Structure. — The  volume  of  air  in  a  water-free 
soil  being  identical  with  the  pore  space,  the  formation 
of  aggregates  of  particles  is  favorable  to  a  large  air 
volume.     The    volume   of   air   in   any   soil,   therefore, 
changes  from  time  to  time;  and  particularly  is  this  true 
of  a  fine-grained  soil,  in  which  the  changes  in  structure 
are  greater  than  in  a  soil  with  large  particles.   A  change 
in  soil  structure  may  greatly  alter  the  volume  of  air  con- 
tained, by  altering  the  pore  space,  thereby  influencing 
the  productiveness.-  Clay  is  most  affected  in  this  way. 

(432) 


THE  SOIL  ATMOSPHERE  433 

297.  Organic  matter. — Organic  matter  being  more 
porous  than  any  size  or  arrangement  of  mineral  particles, 
the  effect  of  that  constituent  is  always  to  increase  the 
volume  of  air.  While  this  is  generally  beneficial  in  a 
humid  region,  it  is  often  very  injurious  in  an  arid  one. 
Unless  sufficient  water  falls  upon  the  soil  to  wash  the 
soil  particles  around  the  organic  matter  and  to  maintain 
a  supply  sufficient  to  promote  decomposition,  the  pres- 
ence of  vegetable  matter  leaves  the  soil  so  open  that  the 


FKI.  llo.    Blade  cultivator,  with  hammock  seat.    For  surface  work. 

capillary  rise  of  moisture  is  interfered  with,  and  the  large 
movement  of  air  keeps  the  soil  dry,  with  the  result  that 
the  portion  of  the  soil  layer  mixed  with  and  lying  above 
the  organic  matter,  is  too  dry  to  germinate  seeds  or 
support  plant  growth. 

298.  Moisture  content. — It  is  quite  evident  that  the 
larger  the  proportion  of  the  interstitial  space  filled  with 
water  the  smaller  will  be  the  quantity  of  air  contained. 
This  does  not  necessarily  mean  that  the  higher  the  per- 

BB 


434        THE   PRINCIPLES    OF    SOIL    MANAGEMENT 

centage  of  water  in  the  soil  the  smaller  the  volume  of 
air,  as  the  amount  of  pore  space  determines  both  the 
water  and  the  air  capacity.  A  soil  with  30  per  cent 
moisture  may  contain  more  air  than  one  with  a  water 
content  of  20  per  cent  because  of  the  tendency  of  mois- 
ture to  move  the  soil  particles  further  apart. 

In  soils  in  the  field,  the  average  diameter  of  the  cross- 
section  of  the  pore  space  is  the  most  potent  factor  in 
determining  the  volume  of  air.  Small  spaces  are  likely 
to  hold  water,  while  the  larger  ones,  not  retaining  water 
against  gravity,  are  filled  with  air. 

In  a  clay  soil,  the  volume  of  air  is  increased,  other 
things  being  equal,  by  the  formation  of  granules,  and 
decreased  by  deflocculation  or  compaction. 

II.     COMPOSITION    OP    SOIL    AIR 

The  air  of  the  soil  differs  from  that  of  the  outside 
atmosphere  in  containing  more  water  vapor,  a  much 
larger  proportion  of  carbon  dioxid,  a  correspondingly 
smaller  amount  of  oxygen,  and  slightly  larger  quantities 
of  other  gases,  including  ammonia,  methane,  hydrogen 
sulphid,  etc.,  formed  by  the  decomposition  of  organic 
matter. 

299.  Analyses  of  soil  air. — The  composition  of  the 
air  of  several  soils,  as  determined  by  Boussingault  and 
Lewy,  is  quoted  by  Johnson  in  the  table  on  the  follow- 
ing page. 

There  are  several  factors  influencing  the  composition 
of  the  soil  air,  those  of  greatest  importance  being  the 
production  and  the  escape  of  carbon  dioxid,  while  of 


COMPOSITION  OF  SOIL  AIR 
TABLE  LXV 


435 


Character  of  soil 

Volume  in  one 
acre  of  soil  to 
depth  of  14  inches 

Composition  of  100  parts 
soil-air  by  volume 

Air 

Carbon 
dioxid 

Carbon 
dioxid 

Oxygen 

Nitro- 
gen 

Sandy  subsoil  of  forest  .  . 
Loamy  subsoil  of  forest  . 
Surface  soil  of  forest  .... 
Clay  soil  

Cu.ft. 
4,410 
3,530 
5,891 
10,310 

11,182 
11,182 
11,783 

11,783 
21,049 

Cu.ft. 
14 
28 
57 
71 

86 
172 
257 

1,144 
772 

0.24 
0.79 
0.87 
0.66 

0.74 
1.54 
2  21 

9.74 
3.64 

19.66 
19.61 
19.99 

19.02 
18.80 

10.35 
16.45 

79.55 
79.52 
79.35 

80.24 
79.66 

79.91 
79.91 

Soil  of  asparagus  bed  not 
manured  for  one  year. 
Soil    of    asparagus    bed 
freshly  manured 

Sandy     soil,     six     days 
after  manuring 

Sandy  soil,  ten  days  after 
manuring  (three  days 
of  rain)  
Vegetable  mold  compost 

less  influence  is  the  excretion  of  carbon  diox-id  and  utili- 
zation of  oxygen  by  plant  roots. 

300.  Production  of  carbon  dioxid  as  affecting  com- 
position.— Although  the  formation  of  carbon  dioxid 
in  the  soil  depends  upon  the  decomposition  of  organic 
matter,  it  is  not  always  proportional  to  the  quantity 
of  organic  matter  present.  The  rate  of  decomposition 
varies  greatly,  and  where  this  is  depressed,  as  is  some- 
times seen  in  muck  or  forest  soils,  the  content  of  carbon 
dioxid  is  low.  A  high  percentage  of  organic  matter  is  in 
itself  likely  to  prevent  a  proportional  formation  of  carbon 
dioxid  by  the  accumulation  of  the  gas  inhibiting  further 
activity  of  the  decomposing  organisms. 


436 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


Ramann  states  that  the  percentage  of  carbon  dioxid 
in  the  soil  air  has  the  following  relations: 

The  carbon  dioxid  increases  with  the  depth. 

In  general  the  percentage  of  carbon  dioxid  rises  and 
falls  with  the  temperature,  being  higher  in  the  warm 
months  and  lower  in  the  cold  months. 


Fio.  116.     Disc  cultivator  fitted  with  fenders. 

Changes  in  temperature  and  air  pressure  change  the 
percentage  of  carbon  dioxid. 

In  the  same  soil  the  content  of  carbon  dioxid  varies 
greatly  from  year  to  year. 

An  increase  of  moisture  in  the  soil  increases  the  per- 
centage of  carbon  dioxid. 

The  amount  of  carbon  dioxid  varies  in  different  parts 
of  the  soil. 

301.  Escape  of  carbon  dioxid  as  affecting  composition. 


FUNCTION   OF    THE   SOIL   AIR  437 

— The  movement  of  carbon  dioxid  from  the  soil  depends 
chiefly  upon  diffusion  into  the  outside  atmosphere. 
The  conditions  governing  diffusion,  which  will  be  dis- 
cussed later  (page  4.39),  therefore  largely  determine 
the  rate  of  loss  of  carbon  dioxid  from  the  soil. 

302.  Effect  of  roots  upon  composition. — The  absorp- 
tion of  oxygen  and  excretion  of  carbon  dioxid  by  roots 
has  a  real,  but  as  yet  unmeasured  influence  upon  the 
composition  of  the  soil  air.   It  is  worthy  of  note,  however, 
that  the  carbon   dioxid  thus  excreted   is  in   a  position 
where  its  aqueous  solution  can  be  of  the  greatest  benefit 
to  the  plant  in  its  solvent  action  upon  the  soil,  as  it  is 
in   direct   contact    with    the   absorbing   portion   of   the 
roots. 

III.      FT.NTTIONS    OK    THE    SOIL    AIR 

Both  carbon  dioxid  and  oxygen  as  they  exist  in  the 
air  of  the  soil  have  important  relations  to  the  processes 
by  which  the  soil  is  maintained  in  a  habitable  condition 
for  the  roots  of  plants.  Deprived  of  these  gases,  the  soil 
would  soon  reach  a  sterile  condition. 

303.  Oxygen.     An   all-important    process   in   the  soil 
is  that   of  oxidation,  because  by  it   the  organic  matter 
that  would  soon  accumulate  to  the  exclusion  of  higher 
plant  life   is   disposed   of.   and   the   plant-food    materials 
arc   brought    into   a   condition    in    which    they    may    be 
absorbed    by    plant-roots.     The    presence   of   oxygen    is 
essential  to  the  life  of  the  decomposing  organisms  and 
to  the  complete  decay  of  organic  matter.    Through  this 
process,  roots  of  past  crops,  as  well  as  other  organic  matter 
that  has  been  plowed  under,  are  removed  from  the  soil. 


438 


THE   PRINCIPLES    OF  SOIL   MANAGEMENT 


The  process  of  decay  gives  rise  to  products,  chiefly 
carbon  dioxid,  that  are  solvents  of  mineral  matter,  and 
leaves  the  nitrogen  and  ash  constituents  more  or  less 
available  for  plant  use. 

Oxygen  is  also  necessary  for  the  germination  of  seeds 
and  the  growth  of  plant-roots.  These  phenomena, 
although  not  involving  the  removal  of  large  quantities 

of  oxygen,  are 
yet  entirely  de- 
pendent upon 
its  presence  in 
considerable 
amounts. 

304.  Carbon 
dioxid.— The 
solvent  action 
of  carbon  dioxid 
is  its  most  im- 
portant func- 
tion in  the  soil.  By  its  solvent  action  it  prepares  for 
absorption  by  plantrroots  most  of  the  mineral  substances 
found  in  the  soil.  Although  a  weak  acid  when  dissolved 
in  water  its  universal  presence  and  continuous  formation 
during  the  growing  season  results  in  a  large  total  effect. 
Carbonic  acid  dissolves  from  the  soil  more  or  less 
of  all  the  nutrients  required  by  plants.  The  amounts  so 
dissolved  are  appreciably  greater  than  those  dissolved 
in  pure  water.  The  constant  formation  of  carbon  dioxid 
by  decomposition  of  organic  matter  keeps  this  solvent 
continually  in  contact  with  the  soil. 

Carbon  dioxid  serves  a  useful  purpose  in  combining 


FIQ.  117.    Hand  cultivator,  or  wheel  hoe,  with 
attachments. 


MOVE\fENTS   OF   THE  SOIL   AIR  439 

with  certain  bases  to  form  compounds  beneficial  to  the 
soil.  Particularly  is  this  the  case  with  calcium  carbo- 
nate, which  is  of  the  greatest  benefit  to  the  soil  in  main- 
taining a  slight  alkalinity  very  favorable  to  the  develop- 
ment of  beneficial  bacteria  and  to  the  maintenance 
of  good  tilth. 

When  combined  as  sodium  or  potassium  carbonate 
in  considerable  quantity,  as  in  certain  alkali  soils,  a  very 
injurious  action  upon  plant-roots,  and  upon  soil-struc- 
ture results.  Upon  plants  it  acts  as  a  direct  poison. 
(See  page  .312.)  The  effect  upon  soil  structure  is  to  de- 
flocculate  the  particles  producing  the  separate  grain  or 
compact  arrangement.  (See  page  116.) 

IV.     MOVEMENT    OF    SOIL    AIR 

There  is  a  constant  movement  of  the  air  in  the  inter- 
stitial spaces  of  the  soil,  and  an  exchange  of  gases  between 
the  soil  atmosphere  and  the  outside  atmosphere,  as  well 
as  a  more  general  but  probably  less  effective,  movement 
of  the  air  out  of,  or  into  the  soil,  as  the  controlling 
conditions  may  determine. 

The  movement  may  be  produced  by  any  one  or  more 
of  the  following  phenomena:  (1)  Gaseous  diffusion. 
(2)  Movement  of  water.  (3)  Change  of  atmospheric 
pressure.  (4)  Change  of  temperature  in  soil  or  atmos- 
phere. (5)  Suction  produced  by  wind. 

306.  Diffusion  of  gases. — The  wide  difference  in  the 
composition  of  soil  and  atmospheric  air  gives  rise  to  a 
movement  of  gases  due  to  a  tendency  for  the  external 
and  internal  gases  to  come  into  equilibrium.  According 


440          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

to  Buckingham,  the  interchange  of  atmospheric  and  soil 
air  is  due  in  large  measure  to  diffusion. 

The  rate  of  movement  of  the  soil  air  due  to  diffusion 
is  dependent  upon  the  aggregate  volume  of  the  interstitial 
spaces,  and  not  upon  their  average  size.  Thus  it  is  the 
porosity  of  the  soil  that  influences  most  largely  the  dif- 
fusion of  the  air  from  it,  and  consequently  the  size  of  the 
particles  is  not  a  factor,  but  good  tilth  permits  diffusion 
to  take  place  more  rapidly  than  does  a  compact  condi- 
tion of  soil,  as  the  volume  of  the  pore  space  is  thereby 
increased.  Compacting  the  soil  in  any  way,  as  by  rolling 
or  trampling,  has  the  opposite  effect. 

306.  Movement  of  water. — As  water,  when  present 
in  a  soil,  fills  certain  of  the  interstitial  spaces,  it  thus 
decreases  the  air   space  when  it    enters  the  soil    and 
increases  it  when  it  leaves.    The  downward  movement 
of  rain-water  produces  a  movement  of  soil  air  by  forc- 
ing it  out  through  the  drainage  channel  below,  while  at 
the  same  time  a  fresh  supply  of  air  is  drawn  in  behind 
the  wave  of  saturation,  as-  the  water  passes  down  from 
the  surface.    The  movement  thus  occasioned  extends  to 
a  depth  where  the  soil  becomes  permanently  saturated 
with  water.    Twenty-five  per  cent  of  the  air  in  a  soil 
may  be  driven  out  by  a  normal  change  in  the  moisture 
content  of  the  soil. 

307.  Changes   in    atmospheric    pressure. — Waves    of 
high  or  low  atmospheric  pressure,  frequently  involving 
a  change  of  .5  inches  on  the  mercury  gage,  cross  the 
continent  alternately  every  few  days.   The  presence  of  a 
low  pressure  allows  the  soil  air  to  expand  and  issue  from 
the  soil,  while  a  high  pressure  following,  causes  the  out- 


FACTORS   AFFECTING   SOIL   AERATION  441 

side  air  to  enter  in  order  to  equalize  the  pressure.  An 
appreciable,  but  not  important  movement  of  soil  air  is 
produced  in  this  way. 

The  size  of  the  interstitial  spaces  is  more  potent  than 
their  volume  in  effecting  soil  ventilation  by  this  and  the 
following  methods. 

308.  Changes  in  temperature. — A  movement  of  soil 
air  may  be  induced  by  a  change  of  temperature  in  the 
atmosphere  or  in  that  of  the  soil  itself.  Changes  in  atmos- 


Fio.    118.     The  hillside  plow.    The  hinged  share  and  moldhoard  permit   con- 
tmiiciii-  plowing  on  one  .tide  of  the  land. 

pheric  temperature  act  in  the  same  way  as  do  changes 
in  atmospheric  pressure;  in  fact,  it  is  the  effect  of  tem- 
perature upon  air  pressure  that  causes  the  movement. 
Like  the  movement  due  to  atmospheric  pressure,  it  is 
not  great :  but  where  the  soil  immediately  at  the  sur- 
face of  the  ground  attains  a  temperature  of  120°  Kahr. 
at  mid-day,  as  occurs  in  the  corn-belt,  the  movement 
must  be  appreciable. 

The  diurnal  change  in  soil  temperature  decreases 
rapidly  from  the  surface  downward,  due  to  the  absorp- 
tion and  slow  conduction  of  heat.  (See  page  400.")  At 
the  Nebraska  Kxperiment  Station,  the  average  diurnal 
range  for  the  month  of  August,  1801,  was  as  follows: 


442         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 
DIURNAL  RANGE  OF  Am  AND  SOIL  TEMPERATURES 

Degrees  Fahr. 

Air  5  feet  above  ground 14.4 

Soil  1  inch  below  surface 17.9 

Soil  3  inches  below  surface 14.8 

Soil  6  inches  below  surface 9.2 

Soil  9  inches  below  surface 6.6 

Soil  12  inches  below  surface 4.3 

Soil  24  inches  below  surface 0.5 

Soil  36  inches  below  surface    0.0 

This  soil  contains  about  50  per  cent  of  pore  space,  in 
the  upper  foot  of  which  40  per  cent  is  normally  filled 
with  water  during  the  summer  months.  This  leaves  518 
cubic  inches  of  air  in  the  upper  cubic  foot  of  soil.  With 
an  increase  in  temperature,  the  air  expands  T£T  in  volume 
for  each  degree  Fahr.  The  average  increase  of  tempera- 
ture is,  in  this  case,  about  11°  Fahr.  for  the  first  foot. 
The  air  exhaled  or  inhaled  by  each  cubic  foot  of  soil 
would  then  be 

518  x  11 

— -rp- =  11.6  cubic  inches. 

491 

As  this  is  slightly  over  2  per  cent  of  the  air  contained 
in  the  upper  foot  of  soil,  and  as  the  movement  below 
that  depth  is  negligible,  the  change  in  composition  at  any 
one  time  is  not  great;  but  this  pumping  effect  is  kept  up 
day  after  day,  although  less  energetically  in  the  cooler 
portion  of  the  year.  In  proportion  as  poor  drainage 
equalizes  the  temperature  it  would  prevent  this  type  of 
circulation.  The  total  effect  assisted  by  diffusion  is  to 
aid  materially  in  ventilating  the  soil.  Owing  to  diffusion 
of  air  in  the  interstitial  spaces,  the  air  expelled  is  dif- 
ferent in  composition  from  that  inhaled. 


MODIFICATION   OF  SOIL   AERATION  443 

309.  Suction  produced  by  wind. — The  movement  of 
wind,  being  almost  always  in  gusts,  alternately  increases 
and  decreases  the  atmospheric  pressure  at  the  surface 
of  the  soil.  There  is  a  tendency,  therefore,  for  the  soil 
air  to  escape  and  for  atmospheric  air  to  penetrate  the 
soil  with  each  change  in  pressure.  The  effect  presumably 
influences  only  the  superficial  air  spaces,  but  it  must  be 
very  frequent  in  its  action.  No  measurements  have 
been  made  and  no  definite  estimate  of  its  effect  can  be 
arrived  at. 


V.     METHODS    FOR    MODIFYING    THK    VOLUME    AND 
MOVEMENT    OF    SOIL    AIR 

The  conditions  that  affect  the  ventilation  of  soils  are: 
(1)  The  volume  and  size  of  the  interstitial  spaces.  (2) 
The  moisture  content.  (3)  The,  daily  and  annual  range 
in  temperature. 

Although  the  size  of  the  interstitial  spaces  does  not 
appear  to  influence  greatly  the  diffusion  of  gases  from 
a  soil,  it  has  a  marked  effect  upon  certain  of  the  other 
processes  by  which  air  enters  and  leaves  the  soil.  A 
sandy  soil,  a  soil  in  good  tilth,  and.  particularly,  a  soil 
composed  of  clods,  permit  of  more  rapid  movement  of 
air  than  does  a  compact  soil. 

While  a  certain  movement  of  air  through  the  soil  i.s 
desirable,  and  indeed  necessary,  for  the  reasons  already 
stated,  a  very  large  movement  is  injurious  unless  there 
is  an  abundant  rainfall.  The  effect  of  air  movement 
through  the  soil  is  to  remove  soil  moisture.  In  a  region 
of  small  rainfall  and  low  atmospheric  humidity,  this 


444          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

may  be  disastrous  if  the  soil  is  not  kept  compact  by 
careful  tillage.  On  the  other  hand,  in  a  humid  region  and 
in  clay  soil,  there  is  likely  to  be  too  small  a  supply  of 
oxygen  for  the  use  of  crops  and  lower  plant  life  unless 
the  soil  is  well  stirred. 

310.  Tillage. — The  ordinary  operations  of  tillage 
influence  greatly  the  ventilation  of  the  soil.  When  a  soil 
is  plowed,  the  soil  at  the  bottom  of  the  furrow  is  exposed 
directly  to  the  air  at  the  surface,  and,  by  the  separation 


FIG.  119.     The  Acme  harrow.    An  efficient  pulverizer  on  clean  soil, 
free  from  stones. 

of  adhering  particles  and  aggregates  of  particles,  air 
is  brought  in  contact  with  particles  that  may  previously 
have  been  completely  shut  off  from  the  air.  It  is  largely 
because  of  its  effect  upon  soil  ventilation  that  plowing 
is  beneficial,  and  the  necessity  for  its  practice  is  greater 
in  a  humid  region  and  upon  a  heavy  soil  than  in  a  region 
of  small  rainfall  and  on  a  light  soil.  The  practice  of  list- 
ing corn,  by  which  the  soil  is  sometimes  left  unplowed 
for  a  number  of  years,  although  in  the  semi-arid  region, 
productive  of  crops  of  sufficient  yield  to  make  them 


d  — 

1  8 


446         THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

profitable,  would  fail  utterly  on  the  heavy  soils  of  a 
humid  region. 

Subsoiling  by  loosening  the  subsoil  increases  the 
ventilation  to  a  greater  depth.  Rolling  and  sub-surface 
packing  both  diminish  the  volume  and  movement  of  air. 
Their  essential  difference  is  in  their  effect  upon  moisture 
rather  than  upon  air.  (See  page  111.)  Harrowing  and 
cultivation  have  the  opposite  effect,  and  both  increase 
the  production  of  nitrates  in  the  soil  by  promoting 
aeration.  The  tillage  which  is  most  beneficial  is  that 
which  increases  the  porosity  of  the  soil,  and  not  the  size 
of  the  interstitial  spaces. 

311.  Manures. — Farm     manures,     lime     and     those 
amendments  that  improve  the  structure  of  the  soil,  have 
to  the  same  degree  a  beneficial  action  upon  soil  aeration. 
By  their  effect  upon  the  physical  condition  of  the  soil, 
they  increase  its  permeability,  and  by  their  action  in  con- 
tributing to  the  production  of  carbon  dioxid  they  stimu- 
late diffusion. 

It  is  chiefly  through  its  effect  in  increasing  the  volume 
of  air  space  in  soils  that  farm  manure  is  injurious  in  light 
soils  of  the  semi-arid  region.  It  may  thus  be  injurious 
as  well  as  beneficial,  if  used  under  certain  conditions. 

312.  Underdrainage. — By  lowering  the  water  table, 
underdrainage  by  means  of  tiles  removes  from  the  soil 
the  water  from  all  but  the  small  capillary  spaces,  and 
leaves  free  to  the  air  the  remainder  of  the  interstitial 
spaces.    There  is  also  a  very  considerable  movement 
of  air  through  the  drains,  and  a  movement  of  air  upward 
from  the  drains  to  the  surface  of  the  soil,  which,  serves 
to  aerate,  to  some  extent,  this  intervening  layer.    The 


PLANT   ROOTS    AND    SOIL   AERATION  447 

aeration  of  the  soil  brought   about   by   underdrainage 
is  one  of  its  beneficial  features. 

313.  Irrigation.— The    influence    of    irrigation    upon 
the  soil  is   much   like  that  of  rainfall.     The   alternate 
filing  and  emptying  of  the  interstitial  spaces  with  water 
and  air  causes  a  very  considerable  change  of  air. 

314.  Cropping. — The  roots  of  plants  left  in  the  soil 
after  a  crop  has  heen  harvested  decay  and  leave  channels 
in  the  soil  through   which  the  air  penetrates.     Below 
the  furrow  slice,  where  the  soil  is  not  stirred  and  where 
it  is  usually  more  dense  than  at  the  surface,  this  affords 
an  important  means  of  aeration.   The  growth  of  legumi- 
nous plants  and  other  deep-rooted  crops  is  in  this  way, 
among  others,  beneficial  to  the  soil.    The  absorption  of 
moisture  from  the  soil  by  roots  also  causes  the  air  to 
penetrate,  in  order  to  replace  the  water  withdrawn. 


F.    HEAT  OF  THE  SOIL 

I.     FUNCTION    OF   THE    HEAT    OF   THE    SOIL    IX    ITS 
RELATION    TO    PLANT    GROWTH 

The  heat  of  the  soil  has  three  general  functions  with 
reference  to  plant  growth.  These  are:  (1)  Biological. 
(2)  Chemical.  (3)  Physical. 

315.  Biological. — Heat  is  the  motive  power  in  plant 
growth.    A  certain  degree  of  heat  is  necessary  for  the 
normal  action  of  all  of  the  functions  of  the  plant.    When 
the  soil,  as  well  as  the  atmospheric  temperature,  passes 
beyond  a  certain  maximum  or  minimum  degree,  growth 
is  inhibited.     These   points   differ  for  different  species 
and  groups  of  plants,   and  they  may  be  different  for 
different  individuals  of  the  same  species.    Somewhere 
between  the  maximum  and  the  minimum  temperature 
which  any  plant  can  withstand  and   still    live,  is    the 
optimum  or  best  temperature  for  growth.    These  rela- 
tions may  be  divided  into  the  following  three  groups.  The 
best  soil  temperature  for:  (1)  Germination.  (2)  Growth 
and  vegetation.    (3)  Proper  activity  of  the  soil  organ- 
isms. 

316.  Germination. — This  takes  place  at  widely  dif- 
ferent  temperatures   for    different    plants.     Ordinarily, 
the   optimum   temperature  for  germination  is  several 
degrees    below  the   optimum    temperature   for  growth 
during  the  average  period  of  vegetation. 

The  range  for  a  few  common  plants  is  shown  in  the 
following  table: 

(448) 


SOIL   TEMPERATURE   FOR   PLANT   GROWTH        449 
TABLE  LXVI 


Temperatures  for  germination  in  degrees 
Fahrenheit 

Minimum 

Optimum 

Maximum 

Melons. 

55-65 

88-100 
75-  90 
75-  85 
75-  i»5 
60-  75 
85-  90 
70-  85 
75-  80 
60     75 
60-  85 

110-120 
90-110 
90-100 
100   110 
100   105 
100-110 
90-100 
85     95 
90-100 
90-100 

Tobacco 

50-60 
45-50 
40-45 
38-45 
36-45 
32-45 
32-40 
32  40 
32-38 

Maize 

Red  clover  and  alfalfa  
Barley  and  vetch  . 

Turnips    

Oats  

Flax  

Rye  

Mustard  

These  figures  show  that  germination  may  take  place 
as  low  as  32°  Fahr.  for  some  seeds,  but  that  the  best 
temperature  is  from  00°  to  90°  Fahr.,  with  the  average 
near  85°.  Few  seeds  germinate  at  temperatures  much 
above  1(X)°  Fahr.  At  temperatures  below  the  optimum, 
the  time  required  is  correspondingly  increased, — as 
shown,  for  example,  by  Nobbe,  who  found  that  musk- 
melon  seeds  required  2iX)  hours  to  germinate  at  ('•(). A0 
Fahr.,  but  at  88°  Fahr.  they  germinated  in  forty-eight 
hours.  The  long  period  may  give  opportunity  for 
certain  fungous  diseases  to  destroy  the  seed. 

317.  (Irowth  and  vegetation. — (Irowth  seldom  takes 
place  below  a  temperature  of  from  40°  to  ">0°  Fahr..  and 
a  much  higher  temperature  is  necessary  for  vigorous 
growth.  Hall  presents  the  following  table,  showing  the 
relation  of  temperatures  to  the  growth  of  some  common 
crops. 

cc 


450         THE  PRINCIPLES   OF  SOIL   MANAGEMENT 
TABLE  LXVII 


Temperature  for  growth  in  degrees  Fahrenheit 

Minimum 

Optimum 

Maximum 

Mustard  

32 
41 
41 
49 
49 
65 

81.0 
83.6 
83.6 
92.6 
92.6 
91.4 

99.0 
99.8 
108.5 
115.0 
115.0 
111.0 

Barley   

Wheat   

Maize  

Kidney  bean  

Melon  

The  figures  in  the  above  two  tables  indicate  that  the 
temperature  of  the  soil  has  a  large  influence  on  germi- 
nation and  growth  of  different  plants.  Those  indivi- 
duals which  require  a  high  temperature  should  not  be 
planted  until  the  soil  attains  the  desired  degree  of  heat. 
If  planted  before  this  point  is  reached,  the  seed  will  be 
slow  to  germinate  and  may  be  destroyed  by  disease. 
If  it  succeeds  in  germinating,  the  growth  will  be  slow 
and  unsatisfactory;  and,  even  if  the  proper  soil  tempera- 
ture is  attained,  the  vigor  of  the  plant  will  have  been  so 
reduced  that  the  maximum  yield  can  not  be  produced. 
The  soil  temperature  also  makes  it  impossible  to  grow 
certain  crops  where  others  thrive.  This  is  a  large  factor 
in  the  distribution  of  crops  and  wild  species  of  plants. 

318.  Activity  of  the  soil  organisms. — The  activity  of 
all  soil  organisms  is  reduced  by  low  temperatures. 
Consequently  those  biological  changes  which  increase 
soil  fertility  are  less  pronounced  during  periods  of  low 
than  during  periods  of  high  temperature.  One  of  the 
most  important  of  these  relations  is  the  formation  of 


SOURCES   OF  HEAT  OF   THE  SOIL  451 

nitrates,  which  takes  place  most  actively  at  a  tempera- 
ture of  80°  to  100°  Fahr.,  and  ceases  at  about  40°  Fahr. 

319.  Chemical  changes. — In  the  soil  chemical  changes 
are  greatly  accelerated  by  a  high  temperature,  and  are 
correspondingly    retarded    by    low    temperature.     But, 
unlike  biological  activity,  they  never  wholly  cease  as  a 
result    of    temperature    changes,    though    the   type   of 
change   in   the   different   compounds    may   be   altered. 
Warm  temperatures  increase  particularly  the  solubility 
of  the  soil  constituents,  by  which  they  are  made  available 
to  plants. 

320.  Physical  changes. — As  a  result  of  temperature, 
physical  changes  are  less  marked  than  the  chemical  and 
biological,  except  when  the  freezing  point  is  reached, 
when  the  soil  moisture  is  solidified  and  renders  nutrition 
of  higher  plants  impossible.    The  movement  of  moisture 
and  gases  through  the  soil  is  greatly  facilitated  by  the 
higher  temperatures  within  the  range  of  plant  growth. 

II.     SOURCES    OF    THE    HEAT    OK    THE    SOIL 

There  are  three  direct  sources  of  heat  which  reach 
the  soil.  These  are:  (1)  Solar  radiation.  (2)  Conduction 
from  the  interior  of  the  earth,  (.'ii  Organic  decompo- 
sition. 

Under  field  conditions,  the  first  of  these  sources  is 
far  the  most  important. 

321.  Solar  radiation. — Solar  radiation  of  heat  reaches 
the  soil  in  three  ways. 

(1)  By  direct  radiation  from  the  sun  in  the  form 
of  sunshine. 


452 


THE   PRINCIPLES   OF   SOIL   MANAGEMENT 


(2)  Indirectly  through  the  radiation  which  is  im- 
parted to  the  atmosphere,  from  which  it  is  radiated  to 
the  soil  or  is  given  up  by  direct  contact  of  the  atmosphere 
with  the  soil.  Clouds  in  the  atmosphere  reflect  back  to 
the  soil  some  heat  which  has  been  received  by  the 

soil  and  is  again 
given  off.  They  may 
serve  as  a  cover  or 
blanket. 

(3)  In  the  spring, 
rain-water  carries  a 
large  amount  of 
heat  into  the  soil. 
The  percolation  of 
warm  spring  rain  is 

FIG.  121.     Mean  annual  sunshine  of  Canada      a  means    of    rapidly 

warming  up  the  soil, 
and  its  strong  in- 
fluence is  shown  by  the  large  quickening  of  growth 
which  follows  such  rainfall. 

322.  Conduction. — Conduction     of     heat     from     the 
interior   of   the   earth   is   negligible   as   an   appreciable 
source  of  soil  heat. 

323.  Organic  decay. — Organic  decay  liberates  heat, 
and  may  be  so  rapid  as  to  greatly  change  the  tempera- 
ture of  the  soil.  This  is  exemplified  by  the  heating  of 
manure  heaps  and  in  the  use  of  the  hotbed.    The  same 
amount  of  heat  is  set  free  by  decomposition  as  would 
result  from  ignition  of  the  material,  but  its  liberation 
is  distributed  over  a  much  longer  period  of  time  accord- 
ing to  the  conditions  for  decay. 


and  the  United  States.  The  figures  indicate  the 
number  of  hours  of  bright  sunshine  in  a  year. 
(From  Bartholomew's  Atlas  of  Meteorology.) 


FACTORS   AFFECTING   SOIL    TEMPERATURE        453 


III.     TEMPERATURE    OF   THK    SOIL 

The  temperature  which  the  soil  in  any  given  position 
will  attain  depends  upon  a  number  of  factors.  The  more 
important  of  these  are  as  follows:  (1)  Heat  supply. 
(2)  Specific  gravity  of  the  soil,  (.'*)  Specific  heat  of  the 
soil.  (4)  Color  of  the  soil.  (5)  Attitude  of  the  surface. 
(6)  Conductivity  of  the  soil.  (7)  Circulation  of  air  above 
the  soil.  (S)  Water-content  of  the  soil. 

324.  Heat  supply. — The  heat  supply  is  obviously 
the  most  direct  factor  contributing  to  the  soil  tempera- 
ture. This  is  reflected  in  the  seasonal,  daily  and  hourly 
variations  in  the  temperature.  The  hourly  variations 
in  temperature  at  a  depth  of  one  foot  below  the  surface 
are  shown  by  the  following  table  and  curves: 

TAMI.K   LXVIII 


1.  Clay  loam,  IVim. 

2.  Loam,  Prim.     .  . 

3.  Silt  loam,  N.C.. 

4.  Sandy  loam,  N.C. 


June  1      Heading*  in  two-hour 


10 


10 


60.5  01 .0  61 .0  61  .3  62.0  62.5  03.0  63.5  63.6  63.6  63.3  63.0 
.    61 .0  IV0.3  W.3  60.6  61 .2  61 .7  62.0  62.1  62.1 162.1 

72.0  70.0  7o.0i6(t.8  70.0  6<>.8  6<».7|6(.).5  69.3  69.0  68.5:68.0 

72. :>  71 .0  69.5  69.8  69.5  (>9.2  »•'.». 0  «HI.O  «>S.S  (W.O  r.7..r)  r>7.2 


Juno  '2    -KradiriK*  in  two-hour 


6         8     j    10 

X         -' 

4          fi 

S         10        M         2 

1.  Clay  loam,  Penn.  02. S  (V2."»  r»2.">  «»2.7  (>H.O  t>4  0  64.5  f>5.0  65.0  <>.">. 0  «>4.S  64.5 

2.  Loam,  Penn (V2.0  61  .S  61 .5  61 .4  61 .3  61  .S  62  2  62. S  63.3  63.3  63.3  63.2 

3.  Silt  loam,  N.C.  .  .  !6H.0.67.3  67.2  67.2  67 .S  fvS. 3  6S.7  6S.S  6S.8  fvS.5  6S.O:67.K 

4.  Sandy  loam,  N.C.  67.0  (Tfi.5  66.0  66.5  67.5  68.2  6S.S  6fl.O  68.7  f>8.3  «>S.O  67.5 


454 


THE  PRINCIPLES   OF  SOIL  MANAGEMENT 


72 

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71 

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I   70 

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S  R7 

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60 

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NOON          6  P.M. 


M.  6A.M.          NOON  8P.M.  M. 


Fio.  122.  Curves  showing  the  daily  range  of  soil  temperature  near  the 
surface  on  soils  of  different  texture  in  Pennsylvania  and  North  Carolina. 
Table  LXVIII. 

The  following  table  and  curves  show  the  average 
mean  monthly  range  in  temperature  of  the  air  and  soil 
at  different  depths  at  Lincoln,  Nebraska  for  a  period  of 
twelve  years. 

TABLE  LXIX.   TEMPERATURE  IN  DEGREES  FAHRENHEIT 


Position 

January 

February 

J3 
§ 

09 

8 

a 
< 

h 

S 

1 

>> 

•t 

I 

< 

September 

October 

November 

December 

1.  Air  
2.  Soil,  1  in  
3.  Soil,  6  in  
4.  Soil,  12  in  
5.  Soil,  36  in  

25.2 
27.3 
28.6 
31.2 
38.5 

24.2 
27.7 
27.8 
30.2 
35.5 

35.8 
38.2 
36.6 
35.4 
35.8 

52.1 
57.5 
53.3 
49.3 
43.8 

61.9 
68.7 
65.1 
60.7 
53.5 

71.0 
78.1 
75.7 
69.9 
61.3 

76.0 
85.1 
81.6 
75.7 
67.4 

74.5 
82.9 
80.1 
75.7 
69.8 

67.6 
73.8 
72.0 
69.2 
67.6 

55.5 
56.7 

57.8 
57.8 
61.3 

38.7 
38.7 
41.5 
44.7 
52.2 

28.3 
31.6 
32.0 
35.2 
43.3 

There  is  a  large  daily  as  well  as  annual  range  in  the 
temperature  of  the  soil.    At  the  surface,  the  range  is 


SUNSHINE  AND  SOIL   TEMPERATURE 


455 


considerably  greater  than  in  the  air  above,  and  this 
excess  extends  to  a  depth  of  nearly  one  foot.  At  greater 
depths  in  the  soil,  the  range  in  temperature  is  less  than 
in  the  air  and  much  less  than  at  the  surface,  and  the 
waves  of  temperature  change  fall  successively  behind 


TIME   IN  MONTHS 

Fid.  123.  Curves  showing  the  mean  monthly  range  in  temperature  of  the 
air,  and  of  the  soil  at  different  depths,  as  given  in  Table  LXIX.  Not«  the  in- 
fluence of  the  rate  of  heat  conduction,  as  shown  by  the  curves. 

those  of  the  atmosphere.  These  variations  are  asso- 
ciated directly  with  the  amount  and  intensity  of  the 
sunshine. 

325.  The  specific  gravity  and  specific  heat. — The 
first  of  these  directly  affects  the  temperature  to  only  a 
small  degree.  The  larger  the  mass,  the  more  hoat  required 
to  change  its  temperature.  Hence,  the  more  dense  the 
soil,  the  more  heat  absorbed  in  each  layer. 

The  specific  heat  of  the  soil  has  a  considerable  in- 
fluence on  its  temperature  and,  because  of  its  marked 


456 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


difference  from  that  of  water,  has  an  important  prac- 
tical bearing.  Drainage  owes  one  of  its  largest  beneficial 
effects  to  this  fact. 

Warington  quotes  from   Lang    the    following    table 
of  specific  heat  of  soil  constituents. 

TABLE  LXX 


Relative  specific  heat  of 

Equal  weights 

Equal  volumes 

Water  

1.000 
0.163 
0.206 
0.260 
0.189 
0.477 
0.233 

1.000 
0.831 
0.561 
0.754 
0.499 
0.587 
0.568 

Ferric  oxide   

Calcium  carbonate  

Magnesium  carbonate  

Quartz,  orthoclase,  granite   

Humus  (peat)  

Clay  

In  the  above  table,  the  specific  heat  of  equal  vol- 
umes is  more  nearly  representative  of  field  conditions 
than  is  that  of  equal  weights.  On  this  basis,  dry  soil 
has  about  one-half  the  specific  heat  of  water;  that  is, 
a  given  amount  of  heat  would  raise  a  mass  of  soil  to 
nearly  twice  the  temperature  that  it  would  the  same 
volume  of  water. 

326.  Color  of  the  soil. — A  dark-colored  soil  absorbs 
heat  much  more  rapidly  than  does  a  light-colored  one, 
and  therefore  warms  up  more  rapidly.  The  effect  of  a 
thin  layer  of  carbon-black  and  chalk  on  the  tempera- 
ture of  dry,  fine  sand,  one  inch  below  the  surface, 
when  exposed  to  the  sun  in  thick  wooden  boxes,  is 
shown  in  the  following  table: 


COLOR  OF   SOIL   AND    TEMPERATURE 
TABLK  LXXI 


457 


Fine  Sand  Soil 

Time  in  minutes  from  start 

0 
I)e«.  F. 

61 

61 

10 

20 

30 

40 

50 

Den.  F. 
81.5 
70.8 

60            70 

1.  Carbon  black  . 
2.  Chalk  (white).. 

Difference.  .  .  . 

DOB.  F. 
65.2 
63.5 

I**.  F. 

71.6 
65.8 

!),•«.  K. 

75.:} 

68.6 

Den.F. 
78.6 
69.5 

Deg.  F.    Dvtf.r. 

84.3      87.0 
T2:2      73.5 

0 

1.7 

5.8 

6.7 

9.1 

10.7      IL'.I       1  :?.:> 

These  figures  agree  with  those  of  Schubler,  who 
found  that,  at  one-eighth  of  an  inch  below  the  surface, 
blackened  soil  attained  a  temperature  from  120  to  1~>° 
Fahr.  warmer  than  the  same  soil  whose  surface  wa.s 
made  white  by  magnesia. 

Humus,  because  of  the  black  or  dark  color  it  im- 
parts to  the  soil,  has  a  large  effect  on  the  soil  ternpora- 


Fio.  124.  Curves  showinK  the  temperature  of  a  dry  «nn«ly  lo.im  »oil, 
covered  by  a  very  thin  layer  of  powdered  chalk  and  carbon  black  respectively, 
after  exposure  in  bright  sunshine. 


458 


THE  PRINCIPLES   OF  SOIL   MANAGEMENT 


ture.  Its  effect  due  to  color  is  reduced  by  the  higher 
water-content  which  such  a  soil  normally  retains. 
(See  page  101.)  Red  soils  absorb  more  heat  than  yellow 
or  gray  ones,  and  yellow  soils  absorb  more  heat  than 
gray  ones. 

327.  Slope  of  the  soil. — A  smooth  surface  absorbs 
more  heat  than  a  rough  or  rigid  surface.  The  effect 
of  the  direction  and  angle  of  slope  on  the  amount  of 
heat  received  from  the  sun  is  shown  by  the  following 
diagram. 


j 


FIG.  125.  Diagram  illustrating  the  influence 
ol  the  slope  of  the  land  surface  upon  the 
amount  of  sunshine  received. 

On  the  21st  of  June,  on  the  42d  parallel,  the  sun- 
beam which  falls  on  a  given  level  area  would  be  dis- 
tributed over  almost  6  per  cent  less  area  when  the  slope  is 
toward  the  sun  at  an  angle  of  20°,  while  on  a  slope  of  20° 
away  from  the  sun  the  same  amount  of  sunshine  would 
fall  upon  16  per  cent  greater  area.  The  area  which 


HEAT   CONDUCTIVITY   OF   THE   SOIL 


459 


sloped  away  from  the  sun  would  also  receive  the  sun's 
rays  for  a  shorter  period  of  each  day.  Wollny  found 
in  Germany  that  the  temperature  of  a  sandy  soil  at 
six  inches  depth  on  a  south  slope  of  30°  averaged  3.1° 
Fahr.  warmer  than  the  corresponding  slope  to  the 
north.  King  found  the  following  differences  in  tempera- 
ture between  the  level  and  an  18°  south  slope,  in  Wis- 
consin, in  July. 

TABLE  LXXII 


First  foot 

Second  foot 

Third  foot 

South  slope,  18  degrees  .  .  . 
Level                   

Degrees  Fahr. 

70.3 
67.2 

Degrees  Fahr. 
68.1 
65.4 

Degrees  Fahr. 
66.4 
63.6 

Difference 

3.1 

2.7 

2.8 

The  north  slope  ordinarily  has  the  most  uniform 
temperature. 

328.  Conductivity. — The  conductivity  of  the  soil 
for  heat  depends  upon  four  factors.  These  are:  (1)  Com- 
position. (2)  Texture.  (3)  Structure.  (4)  Moisture 
content.  The  relative  influence  of  these  factors,  as 
reported  by  Warington  from  the  results  of  Pott,  are 
shown  in  Table  LXXII  I  on  page  4(10. 

Quartz  has  the  largest  power  to  conduct  heat  of 
any  of  the  soil  constituents  studied.  The  effort  of  lime- 
stone and  quartz  stone  is  probably  a  textural  one.  as 
is  shown  by  the  fact  that  the  coarser  the  texture  the 
greater  the  conductivity.  A  compact  soil  conducts 
heat  more  readily  than  a  loose  one.  Hut,  while  a  com- 
pact soil  will  receive  heat  most  rapidly,  it  also  gives 


460         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 
TABLE  LXXIII 


Compo- 
sition 

Texture 

Structure 
Loose  quartz 
powder  =100 

Moisture 
content 

Quartz 
powder 
=100 

Fine 
quartz 
sand 
=  100 

Dry 
quart! 
powder 
=  100 

Loose 

Com- 
pact 

Quartz,  sand  fine 

100.0 
103.6 
105.3 

Quartz  sand,  medium. 

Quartz  sand,  coarse 

Quartz  powder  

100.0 

100.0 
85.2 
90.7 
90.7 

106.7 
92.6 
98.1 
96.4 

201.7 
153.2 
94.3 
155.6 

Chalk  

85.2 

Peat  

90.7 

Kaolin  

90.7 

Clay  

94.1 

Clay  with  limestones 

112.1 

Clay  with  quartz  stones.. 

115.6 

it  up  most  readily.  The  effect  of  the  mulch  is  therefore 
to  maintain  a  more  uniform  soil  temperature.  The 
presence  of  stone  in  the  soil  increases  its  temperature. 
The  movement  of  heat  through  the  soil  is  also  increased 
decidedly  by  the  presence  of  moisture.  Pott  found  that 
when  a  dry  sand  conducted  100  units  of  heat,  the  same 
sand  in  a  moist  state  conducted  174  units,  and  when 
wet,  189  units,  or  nearly  twice  that  for  the  dry  sand.  The 
operation  of  rolling  by  compacting  the  soil  increases 
its  conductivity  for  heat,  and  consequently  its  average 
temperature.  King  found,  as  an  average  of  several 
trials  on  different  soils,  that  at  a  depth  of  1.5  inches, 
rolled  soil  was  3.1°  Fahr.  warmer  than  the  unrolled 
soil,  and  at  a  depth  of  three  inches  the  difference  in 
favor  of  rolling  was  2.9°  Fahr.  In  extreme  cases,  he 
has  found  differences  nearly  three  times  as  great  as 


WATER   CONTENT    AND    TEMPERATURE  461 

the  above  figures  between  the  temperature  of  rolled 
and  unrolled  land.  Rolling  generally  favors  deep 
warming.  The  movement  of  heat  in  the  soil  is  illustrated 
by  the  curves  of  soil  temperature  on  page  455.  The 
change  in  temperature  in  the  subsoil  lags  considerably 
behind  that  at  the  surface,  and  is  also  more  uniform. 

329.  Circulation  of  air. — This  is  due,  first,  to  direct 
conduction  between  the  air  and  the  soil;  and,  second, 
to  the  influence  of  wind  on  evaporation.    Tillage  of  the 
soil,    particularly   in   the   spring,    increases   the   rate   of 
warming,  because  at  that  season  the  air  is  usually  warmer 
than  the  soil,  and,  by  bringing  all  parts  of  the  soil  to 
the  surface  successively,  it  is  warmed  by  contact   with 
the  air  and  by  the  direct  receipt  of  the  sun's  heat.    Wind 
hastens  the  change  in  temperature  of  the  soil  in  either 
direction   by  increasing  the  volume  of  air   with   which 
the  soil  comes  in  contact. 

330.  Water-content.  -  The  water-content   of  the  soil 
is  the  largest  factor,  after  the  heat  supply,  in  determining 
the  temperature  of  the  soil.    This  is  due  to  two  tilings: 

(1)  The  high  specific  heat  of  water  as  compared  to  soil. 

(2)  The    heat    absorbed    in    the   evaporation    of    water. 
The  specific    heat    of    water,   as    compared    with   an 

equal  volume  of  soil,  is  shown  by  the  table  on  pane  4~>(i 
to  be  nearly  twice  as  great.  Consequently,  the  more 
water  a  soil  contains,  the  more  slowly  will  its  tempera- 
ture change  with  a  given  heat  supply.  The  tempering 
influence  of  large  bodies  of  water  upon  adjacent  land 
areas  is  an  example  of  this  fact. 

In  the  evaporation  of  water,  a  large  amount  of  heat  is 
absorbed.     The    vaporization  of  one  pound  of  water  at 


462         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

the  boiling  point  requires  5.3  times  as  much  heat  as  is 
necessary  to  raise  its  temperature  from  the  freezing  point 
to  the  boiling  point.  It  is  this  large  absorption  of  heat 
which  renders  evaporation  such  a  large  cooling  operation. 
The  more  evaporation  which  takes  place  from  the  soil 
moisture,  the  more  will  the  temperature  be  kept  down. 
Any  treatment  which  reduces  evaporation,  such  as  the 
mulch,  will  favor  a  higher  soil  temperature. 

This  influence  of  the  moisture  content  has  given 
rise  to  popular  descriptive  terms,  -such  as  "warm," 
and  "cold"  soils;  "early"  and  "late"  soils.  A  "warm 
soil"  is  one  which  retains  naturally  a  relatively  small 
amount  of  water,  that  is,  soils  of  coarse  texture.  "Cold 
soils,"  on  the  other  hand,  are  those  which  retain  a  rela- 
tively large  amount  of  water,  that  is,  those  of  fine  tex- 
ture. The  difference  in  the  amount  of  heat  required 
to  warm  the  water  contained  in  the  soil,  as  well  as  that 
lost  in  evaporation,  which  is  of  course  greatest  in  the 
soil  containing  most  water,  is  the  source  of  their  normal 
differences  in  temperature. 

An  "early  soil"  is  one  which  retains  a  relatively 
small  amount  of  water.  It  therefore  warms  up  most 
rapidly  under  a  given  heat  supply,  and  is  in-  condition 
to  permit  seeding  earlier  in  the  season.  A  late  soil 
retains  much  water,  and,  consequently,  is  slow  in 
warming  up.  Its  planting  must  therefore  be  deferred 
until  later  in  the  season.  Coarse-textured  soils  are 
"early,"  and  fine-textured  ones  are  "late."  Wollny 
concluded,  from  extensive  experiments,  that  in  summer 
sandy  soils  are  warmest,  followed  by  humus,  lime  and 
loam  soils.  In  winter  this  order  is  reversed. 


MEANS  OF  MODIFYING  THE  SOIL  TEMPERATURE  463 

The  large  effect  of  drainage  on  the  soil  temperature 
is  due  to  these  heat  relations  of  the  soil  moisture.  If 
the  excess  of  water  is  removed  by  evaporation,  it  keeps 
the  soil  unduly  cold.  King  observed  differences  in  tem- 
perature of  from  2.5°  to  12.5°  Fahr.  between  drained  and 
undrained  soil  on  different  days  in  April.  These  results 
are  abundantly  borne  out  by  practical  experience.  The 
removal  of  the  excess  water  by  drainage  conserves  heat. 


IV.     MEANS    OF    MODIFYING    THE    SOIL   TEMPERATURE 

The  means  of  modifying  the  soil  temperature  are 
obvious  from  the  above  principles.  The  practices  which 
may  be  used  for  this  purpose  are: 

(1)  Modification  of  the  texture  and  structure  of  the 
soil  by  appropriate  tillage. 

(2)  Modification  of  the  color 
of   the   soil,  chiefly  through  the 
addition  of  organic  matter. 


Fl«.  126.    The  spading  disc.    Adapted  to  much  more  »tony, 
hard  soil  than  the  solid  disc. 


464          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

(3)  Modification    in    the    moisture    content    by    the 
use  of  mulches,  irrigation,  and  especially  by  drainage, 
where  there  is  an  excess  of  water. 

(4)  The  attitude  of  the  surface  may  be  somewhat 
changed  by  tillage,  especially  in  the  matter  of  rough  or 
smooth  surface.     Of   course,  the  general  slope  cannot 
be  altered. 

(5)  Promotion  of  organic  decay  through  the  addi- 
tion of  organic  matter  to  the  soil,  in  such  a  state  and 
under  such  conditions  as  will  promote  favorable  decay 
by  which  its  heat  may  be  liberated.   The  high  tempera- 
ture attained  in  hotbeds  in  the  winter  and  early  spring 
exemplifies  this  practice.    The   application  of  manure 
under  field  conditions  may  appreciably  alter  the  soil 
temperature,   due  perhaps  to  several  effects.     Wagner 
observed   an  increase  of  5°   Fahr.   as   a  result   of  the 
application   of  twenty  tons  of   manure  per   acre,   and 
during  a  period  of  several  weeks  there  was  an  average 
excess   of    1°   of    temperature   on   the   manured    land. 
Georgeson  observed,  through  a  period  of  twenty  days 
following  the  application  of  different  amounts  of  manure 
in  the  fall,  temperature  differences  amounting  to  .9° 
for  ten  tons,  1.7°  for  twenty  tons,  2.3°  for  forty  tons, 
and  3.4°  for  an  application  of  eighty  tons  per  acre. 

(6)  Construction   of   shelters    may    modify   the   soil 
temperature.     Coldframes    and   greenhouses    make    use 
of  this  principle  by  preventing  the  circulation  of  air  and 
by  entrapping  the  sun's  rays.    Partial  shade  influences 
the  soil  temperature,  usually  producing  a  lower  average 
and  a  greater  uniformity. 


G.    EXTERNAL  FACTORS  IN  SOIL  MANAGEMENT 

In  the  foregoing  chapters,  some  of  the  principles 
underlying  the  management  of  the  soil  have  been 
pointed  out.  In  addition  to  these  are  several  practices 
associated  with  soil  management  resting  upon  the  prin- 
ciples that  have  been  explained,  which  are  so  funda- 
mentally important  as  to  warrant  their  separate  discus- 
sion in  this  connection. 


I.     MKANS    OF    MODIFYING    THK    SOIL 

In  the  art  of  soil  management,  one  has  a  number 
of  practices  which  may  be  used  to  modify  the  soil. 

331.  Summary  of  practices. — The  most  prominent 
of  these  practices  are:  (1)  The  manipulation  of  the  soil 
by  means  of  implements.  (2)  Drainage,  (.'i)  Irrigation. 
(4)  Application  of  amendments,  including  all  forms 
of  organic  materials.  (5)  Application  of  chemical  ma- 
nures, ((i)  Inoculation.  (7)  Rotation.  (S)  Crop- 
adaptation. 

Each  of  these  practices  has  a  primary  function. 
That  of  drainage  is  to  remove  excess  water  from  the 
soil;  of  chemical  manures  to  add  food  elements:  of  inocu- 
lation, to  introduce  organisms;  of  tillage,  to  modify 
the  structure  of  the  soil.  Hut,  in  the  exercise  of  their 
primary  function,  each  practice  also  lias  many  second- 
ary or  indirect  effects  on  the  soil,  which  may  sometimes 
be  more  important  to  the  productive  qualities  of  the 

DD 


466         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

soil  than  its  direct  effect.  This  complex  effect  is  well 
illustrated  by  drainage,  which  not  only  removes  excess 
water  and  admits  air,  but  it  thereby  affects  the  soil 
temperature,  growth  of  organisms  and  the  elaboration 
of  plant-food.  Similarly,  tillage,  first  of  all,  is  designed 
to  alter  the  structure  of  the  soil,  and  through  this  alter- 
ation in  structure,  the  retention  of  moisture,  aeration 
and  root-penetration,  not  to  mention  many  other 


FIG.  127.     Bottom   view  of  a   modern    plow,  showing  the   parts.    1,  share;  2, 
moldboard;  3,  landside;  4,  frog;  5,  brace;  6,  beam;  7,  clevis;  8,  handle. 

relations,  are  changed.  In  fact,  every  practice  which 
may  be  applied  to  the  soil  influences  in  some  degree 
every  phase  of  the  soil  mechanism.  The  relative  promi- 
nence of  these  different  effects  depends  on  the  character 
and  condition  of  the  soil. 

The  application  of  these  various  practices  has  been 
indicated  in  the  foregoing  pages,  in  connection  with 
the  principles  discussed. 

II.     TILLAGE 

Tillage,  or  the  manipulation  of  the  soil  by  means  of 
implements,  is  so  general  in  its  application  and  so 


OBJECTS  OF   TILLAGE  467 

pronounced  in  its  effects,  as  well  as  complex  in  its 
modes  of  operation,  that  it  is  given  a  separate  treatment. 

332.  Objects  of  tillage. — Tillage  rests  upon  three 
primary  objects.  These  are:  (1)  Modification  of  the 
texture  and  structure  of  the  soil.  (2)  Disposal  of  rubbish 
or  other  coarse  material  on  the  surface,  and  the  incor- 
poration of  manures  and  fertilizers  in  the  soil.  (3)  To  de- 
posit seeds  and  plants  in  the  soil  in  position  for  growth. 

The  most  prominent  of  these  objects  is  the  modifi- 
cation  of    the    soil    structure. 
No  perceptible   change  in  the 
soil  texture  can  be  effected  but 
through  changes  in  structure, 
by    which    it   is    made   either 
more  open   or  more  compact. 
Thereby    the     retention     and     FIQ.  128.  Heel  plate  for 
movement    of    moisture     is         in«  the  width  at  the  heci. 

affected,  aeration  is  altered,  the  absorption  and  reten- 
tion of  heat  is  influenced,  the  growth  of  organisms 
is  either  promoted  or  retarded;  through  all  of  these  the 
composition  of  the  soil  solution  is  affected  and,  lastly, 
the  penetration  of  plant  roots  is  influenced.  The  crea- 
tion of  a  soil  mulch  is  simply  a  change  in  the  structure 
of  the  soil  at  such  time  and  in  such  manner  as  will 
prevent  evaporation  of  moisture.  For  this  reason,  it 
is  essential  to  appreciate  the  relation  of  soil  structure  to 
movement  of  moisture  in  managing  the  mulch.  In  fine- 
textured  soils,  where  the  granular  or  crumbly  structure 
is  most  desired,  tillage  may  have  an  important  influ- 
ence on  the  promotion  or  destruction  of  these  granules. 
As  has  been  pointed  out  (page  105),  any  treatment 


468         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

which  increases  the  number  of  lines  of  weakness  in  the 
soil  structure  facilitates  the  action  of  the  moisture 
films  in  solidifying  the  soil  granules.  Tillage  shatters 
the  soil  and  breaks  it  into  many  small  aggregates  of 
particles,  which  may  be  further  drawn  together  and 
loosely  cemented  by  the  further  evaporation  of  moisture. 
The  more  numerous  the  lines  of  weakness  produced, 
the  more  pronounced  the  granulation;  and,  conversely, 
the  fewer  the  lines  of  weakness  which  result,  the  more 
coarse  and  cloddy  the  structure. 


FIG.  129.    The  modern  sulky  riding  plow. 

333.  Implements  of  tillage. — The  number  of  imple- 
ments adapted  to  the  manipulation  of  the  soil  is  very 
large,    and   they   embrace   many   types   and   patterns. 
Many  operations  are  comprehended  by  the  term  tillage. 
It  includes  the  use  of  all  those  implements  which  are 
used  to  move  the  soil  in  any  way  in  the  art  of  crop-pro- 
duction.   It  includes  the  smallest  hand  implements,  as 
well  as  the  largest  traction  implements. 

334.  Effect  on  the  soil.— All  these  operations  may  be 
divided  into  two  groups,  according  to  their  effect  on 


OPERATION   OF   TILLAGE   IMPLEMENTS  469 

the  soil:  (1)  Those  which  loosen  the  soil  structure. 
(2)  Those  which  compact  the  soil  structure.  In  the 
subsequent  paragraphs  of  this  chapter  the  effect  of  the 
more  common  types  of  tillage  implements  on  the  soil 
are  pointed  out  as  a  guide  to  their  selection  for  the  ac- 
complishment of  a  particular  desired  modification. 
For,  good  soil  management  consists,  first,  in  analyzing 
the  soil  conditions,  to  determine  the  change  which 
should  be  effected;  second,  in  the  selection  of  the  im- 
plement or  other  treatment  which  will  most  readily 
and  economically  accomplish  the  object. 

335.  Mode  of   action. — According  to  their   mode  of 
action,   tillage  implements   may  be  divided  into  three 
groups:    (a)    Plows,    (b)  Cultivators,     (c)  Crushers  and 
packers. 

336.  Plows. — The  primary  function  of  a  plow  is  to 
take  up  a  ribbon  of  soil,  twist  it   upon   itself,  and  lay 
it  down  again  bottom  side  up,  or  partially  so.    In  the 
process  two  things  result.     (1)    If  the  soil  is  in   proper 
condition  for  plowing,  it  will  be  shattered  and  broken  up. 
(2)  The  soil  is  inverted,  and  any  rubbish  is  put  beneath 
the  surface. 

337.  Pulverizations. — In  twisting,  the  soil  tends  to 
shear  into  thin  layers,  as  pointed  out   by   King.    These 
layers  are  moved  unequally  upon  each  other,  as.  when 
the  leaves  of  a  book  are  bent,  they  slip  past  each  other. 
The  result  should  be  a  very  complete  breaking  up  of 
the  soil.     How   thorough   the  breaking-up   will   be  will 
depend  upon  (a)  the  condition  of  the  soil,  and  (b)  the 
type  of  plow.    As  to  the  condition  of  the  soil,  there  is 
a  certain  optimum  moisture  content  at  which  the  best 


470        THE  PRINCIPLES    OF    SOIL    MANAGEMENT 

results  will  be  obtained.  Any  departure  from  this 
moisture  content  will  result  in  less  efficient  work.  It 
has  been  said  that,  in  proportion  to  the  amount  of  energy 


Fio.  130.   Sulky  Lister 


required,  the  plow  is  the  most  efficient  pulverizing 
implement  available  to  the  farmer.  The  optimum 
moisture  content  for  plowing  is  indicated  by  that  nicely 
moist  condition  in  which  a  mass  of  the  soil  when  pressed 
in  the  hand  will  adhere  without  puddling,  but  may  be 
readily  broken  up  without  injury  to  the  intimate 
soil  structure.  This  is  a  much  more  critical  stage  for 
fine-textured  soils  than  for  coarse-textured  ones.  Sandy 
soils  are  not  greatly  altered  by  plowing  when  out  of 
optimum  moisture  condition.  On  the  other  hand,  if  a 
clay  soil  is  plowed  when  it  is  saturated  with  water,  it 


TYPES   OF   TILLAGE  IMPLEMENTS 


471 


will  be  thoroughly  puddled,  and  will  dry  out  into  a  hard 
lumpy  condition.  Such  a  structure  requires  a  considera- 
ble time  to  overcome. 

As  to  the  second  factor,  there  are  two  general  types 
of  turning  plows:  (1)  The  common  moldboard  plow. 
(2)  The  disc  plow.  The  mode  of  action  of  the  two  is 
quite  different,  although,  so  far  as  the  soil  is  concerned, 
the  result  is  much  the  same.  The  moldboard  plow 
seems  to  have  a  wider  application  than  the  disc  plow, 
although  both  have  a  particular  sphere  of  usefulness. 

For  any  given  texture  of  soil  and  any  given  soil 
condition,  there  is  a  type  of  plow,  a  shape  of  mold- 


Fio.  131.  Type*  of  coulters.  Lower  right  hand,  knife  roulter:  lower  left 
hand,  rolling  roul'ter;  upper  right  hand,  fin  coulter;  upper  left  hand,  jointer. 
The  last-named  attachment  assists  in  turning  trash  under  the  surface  as  well 
as  to  cut  the  noil. 


472         THE  PRINCIPLES  OF  SOIL    MANAGEMENT 

board,  and  a  depth  of  furrow  slice,  which  are  calculated 
to  give  the  best  results.  This  fact  is  to  be  kept  constantly 
in  mind  in  plowing  soil.  Sod  land  requires  a  different 
shape  of  plow  from  fallow  land,  sandy  land  from  clay 
land.  Rubbish  on  the  surface  may  be  handled  by  one 
plow  and  not  by  another.  Wet  clay  should  have  the 
use  of  a  different  shape  of  plow  from  dry  soil. 

There  are  several  different  shapes  of  plow.  Among 
these  the  most  prominent  types  are  the  moldboard, 
the  disc,  the  hillside  and  the  subsoil. 

Of  the  moldboard  type  there  are  two  general  shapes  : 

(1)  The  long,  sloping  moldboards,  with  little  or  no  over- 
hang, found  on  what  is  called  the  sod  plow.   This  neatly 
cuts  off  the  roots  at  the  bottom  of  the  slice,  and  slowly 
and  gradually  twists  the  soil  over  without  breaking  the 
sod,   and  lays  it  smoothly  up  to  the  previous  furrow- 
slice.    It  is  seldom  desirable   to   completely  invert   the 
soil.    According   to   depth  of  plowing,  the  furrow-slice 
should  be  laid  at  an  angle  with  the  horizontal  of  from 
25°  to  50°,  so  that  the  projecting  edge  of  the  slice  may 
be  worked  down  for  a  seed-bed,  while   the   roots   and 
rubbish  on  the   surface   is  somewhat  uniformly  distri- 
buted through  a  considerable  depth  of  soil,  instead  of 
occupying  a  single  layer  in  the  bottom  of  the   furrow. 

(2)  The  short,  steep  moldboard  with  a  marked  overhang. 
This  is  not  adapted  to  sod  land,  because  it  breaks  up 
the  sod  and  shoots  it  over  in  a  rough,  jagged  manner 
with  uneven  turning.     But  on  fallow  land,  to  which  it 
is  adapted,  it  very  completely  breaks  up  the  soil  and 
throws  it  over  in  a  nearly  level  mellow  mass.    The  pul- 
verizing effect  is  obviously  much  greater  than  with  the 


THE   PLOW    AS   A    TILLAGE   IMPLEMENT 


473 


sod  plow.  Since  the  steep  moldhoard  or  fallow-ground 
plow  exerts  the  most  force  on  the  soil  in  a  given  time 
at  a  given  speed  of  movement,  it  follows  that  if  a 
particular  soil  is  over-wet  it  should  he  plowed  with 
the  sod-plow,  while,  if  it  must  he  plowed  when  too  dry, 
the  fallow-ground  plow  will  be  more  effective, — disre- 
garding the  draft  which  will  probably  be  large  in  the 
latter  case. 


Fio.   132.     Six-gang  plow.    Usually  operated    hy  ."team    engine.     Ailapted    to 
largo,   level   areas  of   uniform  soil,   relatively  free  from   stone. 

There  is  a  general  relation  between  the  width  of  the 
furrow-slice  and  its  depth.  In  general,  it  may  bo  said 
that  this  ratio  is  about  two  in  width  to  one  in  depth. 
The  greater  the  depth,  the  less  in  proportion  may  he 
the  width  of  the  furrow-slice. 

On  clay  soil  in  particular,  there  is  also  a  relation 
between  depth  and  condition.  A  wot  soil  should  bo 
plowed  more  shallow,  other  things  equal,  than  a  dry  soil, 
because  the  puddling  action  is  less.  On  a  dry  soil,  the 
depth  should  be  increased,  to  increase  the  pulverization. 


474         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

Combining  these  principles,  then,  it  may  be  said  that 
if  a  clay  soil  must  be  plowed  when  too  wet,  it  should  be 
plowed  with  a  sod  plow,  and  to  as  shallow  a  depth  as 
is  permissible.  But,  on  an  over-dry  soil,  the  opposite 
conditions  should  be  fulfilled, — that  is,  steep  mold- 
board  and  increased  depth.  Likewise,  on  sandy  soil, 
where  the  aim  is  generally  to  compact  the  structure, 


FIG.  133.    The  modern  garden  seeder.    It  modifies  the  soil  structure 

this   may   be  furthered   by   deep   plowing   with   steep 
moldboard  when  the  land  is  over-wet. 

In  connection  with  this  phase  of  the  subject,  it  is 
important  to  consider  what  Professor  Roberts  called 
the  plow  sole.  That  is,  the  soil  at  the.  bottom  of  the 
furrow  which  bears  the  weight  of  the  plow  and  trampling 
of  the  team, 'and  which,  under  uniform  depth  of  plowing, 
does  not  become  loosened.  In  clay  soil,  especially,  it 
gradually  becomes  more  compact,  in  time  developing 
something  of  a  "hard-pan"  character,  which  is  detri- 


TYPES   OF   PLOWS  475 

mental  to  the  circulation  of  air  and  moisture  and  inter- 
feres with  the  penetration  of  plant  roots.  Consequently, 
occasional  deep  plowing  or  even  subsoiling  is  recom- 
mended to  break  up  this  unfavorable  soil  structure, 
commonly  called  the  "  plow  sole."  There  is  less  tendency 
for  the  disc  than  the  moldboard  plow  to  form  the  "sole." 
The  hillside  plow  is  a  modified  form  of  the  mold- 
board  plow,  which  has  a  double  curvature  to  the  mold- 
board,  so  that  it  is  essentially  two  plows  in  one.  This 
swings  on  a  swivel 
in  such  a  way  that 
it  may  be  locked 
on  either  the  right 
or  the  left  side.  It 
removes  the  neces- 
sity Of  plowing  in  FIG.  134.  Berry  ho<>  or  ri<lKcr  For  rW  t. !!««»• 
beds  and  b\f  Per-  °'  h*™**-  v'nwi  »"d  low-he«ded  trt-es. 

mitting  all  of  the  work  to  be  done  from  one  side,  enables 
the  plowman  to  lay  the  furrow  slices  in  one  direction. 
On  the  hillside  this  direction  is  down  the  slope,  because 
of  the  greater  ease  in  turning  the  soil  in  that  direction. 
It  also  removes  the  difficulty  of  pulling  up  and  down  the 
hill.  There  is  another  type  of  compound  moldboard 
plow  designed  to  eliminate  "  dead  furrows"  and  "  back 
furrows."  The  former  is  developed  by  turning  the  last 
furrow  slices  of  two  lands  in  opposite  directions,  thereby 
leaving  a  gulley  between  which,  by  reason  of  its  fre- 
quent unproductive  character,  is  termed  the  "dead 
furrow."  The  back  furrow  consists  of  two  furrow  slices 
thrown  together,  usually  forming  a  ridge  more  productive 
than  the  average  of  the  land. 


476         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

The  disc  plow  is  essentially  a  large  revolving  disc 
set  at  such  an  angle  that  it  cuts  off  and  inverts  the  soil, 
at  the  same  time  pulverizing  it  quite  effectively  after 
much  the  same  manner  as  the  moldboard  plow.  One 


FIG.  135.    Disc  plow. 


advantage  claimed  for  it  is  its  lighter  draft  for  the  same 
amount  of  work  done,  because  it  has  rolling  friction  in 
the  soil  instead  of  sliding  friction.  In  practice,  it  appears 
to  be  especially  effective  on  very  dry,  hard  soil  and  in 
turning  and  covering  rubbish. 

338.  Covering  rubbish. — The  secondary  function  of 
the  plow  is  to  cover  weeds,  manure  and  rubbish  which 
may  be  upon  the  surface.  This  also  the  turning  plow 
does  very  effectively.  The  cutting  and  turning  of  the 
sod,  rubbish  and  weeds  is  facilitated  by  several  attach- 
ments. These  are:  (1)  Coulters.  (2)  Jointers.  (3)  Drag- 
chains.  There  are  several  types  of  coulters.  Blade 
coulters  are  attached  to  the  beam  or  to  the  share  in  such 
a  manner  as  to  cut  the  furrow  slice  free  from  the  land 
side.  They  should  be  adjusted  so  as  to  cut  the  soil  after 


THE   PLOW   FOR    COVERING    RUBBISH  477 

it  has  been  raised  and  put  in  a  stretched  condition,  when 
the  roots  are  most  easily  severed.  This  position  is  a 
little  back  of  the  point  of  the  share.  A  knife  edge 
attached  to  the  share  is  commonly  culled  a  fin  coulter. 
A  jointer  is  a  miniature  moldboard  attached  to  the 
beam  for  cutting  and  turning  under  the  upper  edge  of 
the  furrow  slice,  so  that  a  neat,  clean  turn  is  effected 
without  the  exposure  of  a  ragged  edge  of  grass  which 
may  continue  growth.  This  is  used  chiefly  on  sod  land. 
A  drag-chain  is  an  ordinary  heavy  log-chain,  one  end 
of  which  is  attached  usually  to  the  central  part  of  the 
beam,  and  the  other  to  the  end  of  the  double  tree  on 
the  furrow  side,  and  with  enough  slack  so  that  it  drags 
down  the  vegetation  on  the  furrow  slice  just  ahead  of 
its  turning  point.  It  is  used,  primarily,  in  turning  under 
heavy  growths  of  weeds  or  green-manure  crops. 

There  is  a  third  type  of  plow,  the  so-called  subsoil 


FIG.    136.     The  orchard  disc.     Adjustable  and  suited   to  working  rlioe  up  to 
low -headed  trees 


478         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

plow.  The  purpose  of  this  implement  is  to  break  up 
and  loosen  the  subsoil  without  mixing  the  material 
with  the  soil.  It  consists  essentially  of  a  small  mole- 
like  point  on  a  long  shin.  This  implement  is  drawn 
through  the  bottom  of  the  furrow,  and  fractures  and 
loosens  the  subsoil  to  a  depth  of  eighteen  inches  or  two 
feet.  It  is  often  useful  on  soils  having  a  dense,  hard 
subsoil,  but  its  use  requires  the  exercise  of  judgment, 
as  the  process  may  prove  very  injurious  if  done  out  of 
season.  As  a  general  rule,  it  is  best  to  use  the  subsoiler 
in  the  fall  when  the  subsoil  is  fairly  dry,  and  in  order  that 
the  subsoil  may  in  a  measure  be  recompacted  by  the 
winter  rain.  Spring  subsoiling  is  seldom  advisable  in 
humid  regions,  owing  to  the  danger  of  puddling  the  sub- 
soil or  the  possibility  of  its  remaining  too  loose  for  best 
root  development,  if  performed  when  the  subsoil  is  too 
dry  to  puddle. 

339.  Cultivators. — There  are  more  types  of  cultiva- 
tors than  of  any  other  form  of  soil-working  implements. 
These  may  be  grouped  into:  (1)  Cultivators  proper. 
(2)  Leveler  and  harrow  type  of  cultivators.  (3)  Seeder 
cultivators.  These  implements  agree  in  their  mode 
of  action  on  the  soil,  in  that  they  lift  up  and  move  it 
sidewise  with  a  stirring  action  which  loosens  the  struc- 
ture and  cuts  off  weeds,  and  to  a  slight  degree  covers 
rubbish.  However,  the  action  is  primarily  a  stirring 
one,  and,  in  general,  it  is  much  more  shallow  than  that 
of  the  plow.  One  important  fact  should  be  kept  in  mind 
in  cultural  operations,  especially  just  following  the  plow. 
That  is,  to  do  the  work  when  the  soil  is  in  the  right 
moisture  condition.  Particularly  is  this  true  in  the 


TYPES   OF   CULTIVATORS 


479 


pulverization  following  the  plow.  Plowing,  if  it  be  prop- 
erly done,  leaves  the  soil  in  the  best  possible  condition 
to  be  pulverized.  It  is  properly  moistened,  and  if  the 
clods  are  not  shattered  they  are  reasonably  frail  and  may 
be  much  more  readily  broken  down  than  when  they  are 
permitted  to  dry  out.  In  drying,  they  are  somewhat 
cemented  together  and  thereby  hardened.  Not  only  is 
it  desirable  in  almost  all  cases  to  take  advantage  of  this 


Fio.    137.     "Sweeps"    uned   extensively    in    the   southern    atatex.    particularly 
for  shallow  cultivation  of  cotton  and  corn.    (Hartley.! 

condition  of  the  soil,  but  the  leveling  and  pulverizing 
of  the  soil  reduces  drying  and  improves  the  character 
of  the  seed  bed. 

340.  Cultivators  proper.  -There  is  a  groat  va- 
riety in  types  and  patterns  of  cultivators.  They  may  be 
divided  into:  (a)  Large  shovel  forms.  (M  Small  shovel 
forms.  The  former  have  a  few  comparatively  large 
shovels  set  rather  far  apart,  which  vigorously  tear 
up  the  earth  to  a  considerable  depth  and  leave  it  in 
large  ridges.  There  is  a  lack  of  uniform  action,  and 


480          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

the  bottom  of  the  cultivated  portion  is  left  in  hard 
ridges.  Such  implements  are  now  much  less  used  than 
formerly,  and  may  be  considered  to  supplant  in  a  meas- 
ure the  use  of  the  plow,  where  deep  working  without 
turning  is  desired.  Some  of  the  wheel-hoes  used  in  or- 
chard tillage  belong  to  this  type.  The  old  single  and 
double  shovel -plows  are  earlier  types  of  the  same 
implement. 

The    small    shovel-cultivators    have    very    generally 
supplanted  the  large  shovel  type  in  most  cultural  work. 


Fio.  138.    Broadcast  seeder,  which  also  cultivates  the  soil. 

The  decrease  in  size  of  shovels  is  made  up  by  the  great 
increase  in  number.  Ordinarily  they  operate  shallow, 
but  very  thoroughly  and  uniformly.  They  are  now  much 
preferred  in  all  inter-tillage  work  for  eradication  of 
small  weeds  and  the  formation  of  a  loose  surface  mulch. 
A  modification  from  these  in  shape  of  shovel  is  the 
sweep,  much  used  in  the  southern  states,  especially 
in  cotton-growing.  It  consists  of  broad  blunt  knife- 
like  blades,  which  pass  along  a  few  inches  beneath  the 
surface  of  the  soil  and  raise  it  an  inch  or  two,  then 
permit  it  to  drop  back  in  place  in  a  much  broken  con- 
dition. It  works  best  on  soil  relatively  free  from  stone. 


CULTIVATORS    PROPER 


481 


In  addition  to  being  a  good  implement  to  form  a  shallow 
mulch  and  keep  the  surface  level,  it  is  very  effective  as 
a  weed-killer. 


Fio.  139.     Small,  one-horse  grain  drill  for  seeding  in  standing  corn.    Its  use 
is  equivalent  to  cultivation. 

Another  classification,  which  has  less  relation  to 
utility  than  to  the  convenience  and  comfort  of  the 
operation,  is  based  on  the  presence  or  absence  of  wheels. 
There  is  a  strong  movement  toward  the  use  of  wheel- 
cultivators,  carrying  a  seat  for  the  operator.  These 
have  a  wider  range  of  operation  as  to  depth  and  facility 
of  movement  than  have  the  cultivators  without  wheels. 

Still  further,  there  is  the  distinction  of  shovels  from 
discs.  Discs  are  used  on  the  larger  cultivators;  seldom 
on  the  small  ones. 


Fio.  140.    Meeker  disc  pulveriser.    See  altu  Fig    37. 


EE 


482         THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

Cultivators  are  also  adapted  to  till  one  or  more  rows 
at  a  time. 

341.  Leveler  and  harrow  type  of  cultivator. — In 
this  group  come  the  spike-toothed  harrow,  smoothing 
harrow,  the  spring-toothed  harrow,  disc  harrow,  spading 
harrow,  weeders  and  the  Acme  harrow. 

The  spike-toothed  harrow  is  essentially  a  leveling 
implement,  adapted  to  very  shallow  cultivation  of  loose 
soils.  It  is  also  something  of  a  cleaner,  in  that  it  picks 


Fio.  141.     Plow  for  loosening  beets  and  other  root  crops. 
Cultivates  the  soil  deeply. 

up  surface  rubbish.  The  spring-toothed  harrow  works 
more  deeply  than  the  spike-toothed  harrow,  and  can  there- 
fore be  used  in  many  situations  to  which  the  latter  is 
not  adapted.  In  working  down  cloddy  soil  it  brings  the 
lumps  to  the  surface,  where  they  may  be  crushed. 
The  disc  harrow  depends  for  its  primary  advantage 
upon  the  conversion  of  sliding  friction  into  rolling 
friction.  Its  draft  is,  therefore,  less  for  the  same  amount 
of  work  done.  It  has  a  vigorous  pulverizing  action  simi- 
lar to  the  plow,  and  more  so  than  shovel-cultivators. 


LEVELERS  AND  HARROWS  483 

Disc  implements  are  not  adapted  to  stony  soil,  whereas 
toothed  forms  are  as  effective  here  as  on  soil  free  from 
stone,  so  long  as  the  stones  are  not  large  enough  to  collect 
in  the  implement.  On  the  other  hand,  on  land  full  of 
coarse  manure,  sod,  etc.,  the  disc  implement  is  the 
more  efficient.  The  spading  harrow  (cutaway  disc)  is 
very  little  different  from  the  disc  harrow,  except  that 
it  takes  hold  of  the  soil  more  readily.  A  recent  attempt 
to  accomplish  a  large  amount  of  pulverization,  and  with 


FIG.    142.     Riding   cotton-  anil   corn-planter      It    alxo  cultivates   the  anil. 

greater  uniformity,  is  represented  by  the  double-disc 
implements.  In  these  implements  there  are  two  sets 
of  discs,  one  set  in  front  of  and  zig-zagged  with  the 
other,  and  also  adjusted  to  throw  the  soil  in  opposite 
directions. 

Weeders  are  a  modified  form  of  the  spring-toothed 
harrow,  adapted  to  shallow  tillage  of  friable,  easily 
worked  soil,  where  the  aim  is  to  kill  weeds  and  create 
a  thin  surface  mulch.  They  are  wide  and  are  fitted  with 
handles,  and  therefore  stand  intermediate  between 


484         THE  PRINCIPLES  OF  SOIL   MANAGEMENT 

cultivators  proper  and  harrows.    They  are  much  used 
for  the  intertillage  of  young  crops. 

The  Acme  harrow  consists  of  a  series  of  twisted 
blades  which  cut  the  soil  and  work  it  over.  They  are 
most  useful  in  the  latter  stages  of  pulverization  on  soil 
relatively  free  from  stone.  The  Meeker  harrow  is  a 


Fio.  143.    Stubble  digger  used  to  fit  light,  mellow  soils  for  seeding. 

modified  form  of  disc,  used  primarily  for  pulverization. 
It  consists  of  a  series  of  lines  of  small  discs  arranged 
on  straight  axles,  and  is  especially  adapted  to  breaking 
up  hard,  lumpy  soil.  In  this  particular,  it  may  be  con- 
sidered to  belong  to  the  third  set  of  implements,  the 
clod  crushers.  But,  as  compared  with  the  roller  on  hard 
soil,  it  is  more  efficient. 

342.  Seeder    cultivators. — Many    implements    used 


SEEDER    CULTIVATORS 


485 


primarily  for  seeding  purposes  are  also  cultivators, 
and  their  use  is  equivalent  to  a  cultivation.  The  grain 
drill  is  a  good  example  of  this  group.  It  is  essentially  a 
cultivator — either  shoe  or  disc — adapted  to  depositing 
the  grain  in  the  soil  at  the  proper  depth.  All  types  of 
planters  which  deposit  the  grain  in  the  soil  have  a  similar 
action  on  the  structure  of  the  soil.  The  ordinary  two- 
row  maize  planter,  the  potato  planter,  etc.,  while  of 
low  efficiency,  as  cultivators,  still  have  an  effect  which 


Fio.  144.     Grain  drill  with  either  hoes  or  disc*,  and  having 
fertilizor-spreadinK  attachment . 

is  measureable.  This  action  is  well  seen  in  the  lister, 
used  for  planting  maize,  by  which  the  groin  is  deposited 
beneath  the  furrow,  which  is  filled  by  cultivation  after 
the  grain  is  up.  The  lister  is  generally  used  without 
previously  plowing  the  ground,  and  its  use  is  limited 
to  regions  of  low  rainfall  where  the  soil  is  aerated  by 
natural  processes.  Lately,  plowed  ground  listers  have 
been  introduced,  which  combine  the  advantages  of  deep 
planting  with  proper  preparation  of  the  soil. 

There  is  also  a  very  considerable  tillage  action  in 


486 


THE  PRINCIPLES  OF  SOIL   MANAGEMENT 


many  harvesting  implements.    The  potato-digger,   for 
example,  very  thoroughly  breaks  up  and  cultivates  the 


Fin.  145.  Corn  planter;  also  compacts  the  soil  over  the  seed  and  establishes 
capillarity  with  the  lower  soil,  thus  bringing  more  moisture  in  contact  with 
the  seed. 

soil,  which  process  is  one  important  reason  for  the 
general  high  yield  of  crops  following  the  potato  crop. 
Bean-harvesters  and  beet-looseners  also  have  a  similar 
action  on  the  soil. 

343.  Packers  and  crushers. — These  may  be  divided 
into  two  groups:  (a)  Those  implements  which  aim  to 


Flo.  146.     Scotch    chain    harrow.     A   good   pulverizer  and  very  effective   on 
pastures  in  breaking  up  and  spreading  "droppings." 


PULVERIZERS   AND   PACKERS  487 

compact  the  soil.  (6)  Those  whose  primary  purpose 
is  to  pulverize  the  soil  by  crushing  the  lumps.  Both 
sets  of  implements  have  something  of  the  same  action 
on  the  soil.  That  is  to  say,  any  implement  which  com- 
pacts the  soil  does  a  certain  amount  of  crushing;  and, 
conversely,  any  implement  which  crushes  the  soil  does 
some  compacting. 

344.  Rollers. — The  type  of  the  first  group  is  the  solid 
or  barrel  roller,  which  by  its  weight  aims  to  force  the 
particles  of  soil  nearer  together  and  to  level  the  surface. 


Fio.  147.     The  bar  roller  and  pulveriser. 

The  smaller  the  diameter  in  proportion  to  its  weight, 
the  greater  the  effectiveness  of  the  roller.  Its  draft  is 
correspondingly  greater.  As  a  crusher,  the  roller  is 
relatively  inefficient  on  hard,  lumpy  soil,  because  of  its 
large  bearing  surface.  Lumps  are  pushed  into  the  soft 
earth  rather  than  crushed. 

It  should  be  mentioned  that  there  is  one  condition 
where  the  roller  is  effective  in  loosening  up  the  soil 
structure.  This  is  on  fine  soil  on  which  a  crust  has 
developed  as  a  result  of  light  rainfall.  Here  the  roller 
may  break  up  the  crust  and  restore  a  fairly  effective 
soil  mulch. 


488         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

Another  form  of  roller  is  the  sub-surface  packer.  One 
type  of  this  implement  consists  of  a  series  of  wheels  with 
narrow  V-shaped  rims,  which  press  into  the  soil  and  com- 
pact it,  while  leaving  the  surface  loose.  (Fig.  67.)  They 
are  designed  primarily  to  level  the  land  after  plowing, 
and  to  bring  the  furrow  slices  close  together  and  in  good 
contact  with  the  subsoil,  in  order  to  conserve  moisture 


Fio.  148.     Potato  digger  which  is  also  very  effective  in  stirring  the  soil. 

and  promote  decay  of  organic  material,  which  may  be 
plowed  under.  This  implement  has  been  developed 
chiefly  in  semi-arid  and  arid  sections  of  country  where 
the  conservation  of  moisture  is  especially  important, 
but  they  might  well  have  a  much  larger  use  for  the  same 
purpose  in  those  sections  of  the  country  which  are  sub- 
ject to  late  summer  and  fall  droughts.  While  compacting 
the  soil,  these  implements  leave  a  mulch  behind. 


WEEDS  AND   THEIR   CONTROL  489 

345.  Clod-crushers. — The   aim   of  these  implements 
is  to  break  up  lumps.    As  to  mode  of  action,  there  are 
several  forms.    The   bar  roller  and   the  "clod-crusher" 
(see  Fig.  71)  concentrate  their  weight  at  a  few  points, 
and  are  open  enough  so  that  the  fine  earth  is  forced  up 
between  the  bearing  surfaces.    They  are  very  effective 
in  reducing  lumpy  soil  to  comparatively  fine  tilth.  They 
have  very  little  leveling  effect  further  than  the  breaking 
down  of  lumps. 

The  planker,  drag  or  float,  variously  so-called,  con- 
sists essentially  of  a  broad,  heavy  weight  without  teeth, 
which  is  dragged  over  the 
soil.  The  lumps  are  rolled 
under  its  edge  and  ground 
together  in  a  manner  which 
very  effectively  reduces  their  FIO.  149.  Float  or  smoother  mad* 
size.  At  the  same  time,  the  of  planks, 

soil  is  leveled,  smoothed,  and,  to  a  degree,  compacted. 
It  may  well  be  used  in  the  place  of  the  roller  as  a  pul- 
verizer, on  many  occasions.  It  is  constructed  in  many 
forms. 

III.     OTHER    PHASES    OF    TILLAGE    OPERATION 

In  addition  to  the  modification  of  tood.  moisture, 
air  and  heat  of  the  soil,  through  changes  in  its  structure 
as  a  result  of  tillage  and  other  cultural  practices,  other 
important  soil  conditions  may  be  changed.  Two  of  the 
most  important  of  these  are:  (1)  The  destruction  of 
weeds.  (2)  The  control  of  erosion. 

346.  Weeds  in  their  relation   to   crop-production.— 
A  weed  has  been  defined  as  a  plant  out  of  place.    By 


490         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

this  definition  any  plant  which  grows  where  it  is  not 
desired  is  a  weed. 

347.  Objectionable  qualities  of  weeds. — Weeds   are 
objectionable  for  several  reasons.    Some  of  the  objec- 
tionable effects  of  weeds  are:    (1)  They  may  remove 
moisture  needed  by  the  crop.    (2)  They  may  use  food 
needed  by  the  crop.    (3)  They  usurp  the  light  and  heat 
supply.    (4)  They  interfere  with  tillage  and  harvesting, 
operations,  and  perhaps  also  with  the  planting  of  the 
following  crop.    (5)  They  leave  the  soil  in  a  condition 
unfavorable  to  the  growth  of   the  following  crop.     (6) 
They  decrease  the  value  of  the  crop  by  introducing  im- 
purities   which   are   both    injurious    and   expensive  to 
eliminate. 

348.  The  control  of  weeds. — The  control  of  weeds 
depends  on  their  character  and  habits  of  growth.    Each 
situation  develops  its  own  peculiar  crop  of  weeds.   They 
arise  as  a  result  of  the  character  and  condition  of  the 
soil,  and  the  character  and  habits  of  the  regular  crop. 
It  is  a  type  of  the  natural  association  of  plants.    In  the 
wheat  fields  of  the  Northwest,  mustard  is  troublesome; 
in  maize,  it  may  be  quack-grass,  sonchus,  daisy  or  morn- 
ing-glory.  In  meadows,  it  may  be  the  thistle,  yarrow  or 
daisy.    These  weeds  gain  a  foothold  because  their  cycle 
of    growth   so    closely    corresponds   with    that   of    the 
crop.     According  to  the  character  and  occurrence  of  the 
weed,  one  of  two  methods  of  control  or  eradication  must 
be  employed:    (a)  If  its  propagation  is  dependent  on 
seed-production,   then  seed-production  should   be  pre- 
vented. (&)  If  propagated  vegetatively,  then  the  develop- 
ment of  the  aerial  portion  must  be  prevented  for  a 


WEEDS   AND   THEIR   CONTROL 


491 


sufficient  time  to  kill  the  root  or  other  propagative  parts. 
In  this  direction,  much  may  be  accomplished  through 
change  in  the  rotation  and  in  cutting  the  weeds  at  the 
proper  time.  But  much  may  be  accomplished  by  tillage. 
This  may  be  largely  accomplished  in  tillage  for  other 
purposes.  Some  of  the  practices  which  aid  the  process 
are: 

(1)  Early  and  frequent  tillage.  Weeds  are  most 
easily  killed  when  young.  Soon  after  the  seed  has  ger- 
minated, they  are  most  delicate.  Stirring  the  soil  at  this 
period  may  so  change  their  relation  to  it  as  to  cause 


Fio.  150.     Eroeion  on  a  gravelly  hillside. 


492 


THE  PRINCIPLES  OF  SOIL  MANAGEMENT 


their  death.  Tillage  in  hot,  dry  weather  is  especially 
effective  in  killing  most  weeds.  They  soon  dry  out  from 
lack  of  moisture. 

(2)  Small-toothed  implements  which  very  thoroughly 
stir  the  soil  are  more  effective  in  killing  small  weeds 
than  are  large  shovels  which  may  slide  past  the  weed. 
Thorough  stirring  of  the  soil  is  the  essential  point  to 
be  aimed  at. 

(3)  Where  weeds  are  beyond  the  reach  of  the  culti- 
vator, as  in  the  row  in  maize  that  has  reached  a  con- 


FIG.  151.    Terracing  to  prevent  erosion  of  hillside. 

eiderable  size,  they  may  often  be  killed  by  covering 
with  soil  by  use  of  large  shovels. 

Shading  by  a  rapid-growing  leafy  crop  and  spraying 
with  chemicals  for  some  species  are  also  effective  aids 
in  weed  control. 

349.  Erosion. — Erosion  is  often  a  serious  menace  to 
the  productiveness  of  the  soil.  It  may  result  from  two 
causes:  (1)  The  action  of  running  water.  (2)  The  action 
of  wind.  The  soil  is  removed  and  causes  injury  to  the 
productiveness  of  the  land,  first,  by  carrying  away  the 


494         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

most  fertile  portion;  second,  by  such  changes  in  the 
physical  condition  of  the  land  as  greatly  interferes  with 
all  cultural  operations.  This  is  especially  true  where 
large  gullies  are  formed,  as  happens  on  some  soil  types, 
or  where  ridges  and  mounds  are  formed  by  wind  action. 
In  some  sections  and  on  certain  classes  of  soil,  wind 
erosion  is  most  serious;  notably  in  dry  regions  of  high 
winds.  Under  other  conditions,  erosion  by  water  is 
most  serious. 

•  350.  Erosion  by  water. — This  type  of  erosion  is  a 
function  of  flowing  water.  It  therefore  occurs  almost 
entirely  on  sloping  land.  The  exception  is  where  the 
Boil  is  underlain  by  a  stratum  of  fine  sand  which  flows 
with  the  water  when  saturated.  The  removal  of  sand 
below  permits  the  soil  to  cave  down.  As  has  been  noted 
in  another  connection,  erosion  is  greatly  increased  by 
material  carried  by  the  water  and  which  becomes  its 
tool.  Some  of  the  most  effective  practices  for  the  con- 
trol of  this  type  of  erosion  are:  (1)  Deep  plowing  on 
heavy  soil,  by  which  a  larger  part  of  the  rainfall  is 
absorbed  and  retained.  (2)  Increased  granulation  of 
the  soil,  which  may  be  produced  by  the  means  explained 
on  page  104.  The  absorptive  power  and  water  capacity 
of  the  soil  is  thereby  increased  so  that  there  is  a  less 
amount  to  flow  away.  (3)  Addition  of  organic  matter, 
which  not  only  aids  granulation,  but  binds  the  soil 
together.  It  also  increases  the  water  capacity  of  the 
soil.  (4)  Underdrainage  reduces  erosion  where  the  soil 
is  saturated  with  water.  Instead  of  its  flowing  away 
violently  in  rills,  it  is  gradually  removed  in  the  drainage 
channels,  which  are  not  subject  to  erosion.  (5)  Various 


EROSION 


495 


protective  coverings  and  binding  materials  may  be  kept 
on  the  soil.  The  most  effective  of  these  are  fine-rooted 
crops,  which  not  only  hold  the  soil  togther,  but  protect 
It  against  the  force  of  the  water.  In  those  sections  where 


Flo.  153.     Characteristic  erosion  of  loess.   The  binding  power  of  roots  ia  illus- 
trated by  the  tree  roota  at  the  surface. 


496         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

it  thrives,  blue  grass  is  permitted  to  occupy  those  areas 
of  the  hillside  most  subject  to  erosion.  Trees  afford  a 
similar  protection  and  are  valuable  in  reclaiming  eroded 
land.  It  is  a  general  custom  to  retain  in  some  cover- 
crop  those  steep  areas  of  land  most  subject  to  erosion. 

(6)  Contour  farming,  that  is,  the  performance  of  all 
tillage  operations  around  the  hill  at  a  uniform  level, 
instead  of  up  and  down  the  slope,  creates  a  succession 
of  small  ridges  which  hold  the  water,  to  a  certain  extent. 

(7)  Side-hill  ditches  are  employed  where  contour  farm- 
ing does  not  create  sufficiently  large  ridges  to  hold  the 
water.    The  two  are  usually  combined.    These  side-hill 
ditches  are  usually  given  a  small  grade  along  the  face 
of  the  slope,  to  gradually  carry  away  the  water.    (8) 
Terracing  is  preferred  in  some  sections  as  a  method  to 
prevent  erosion  as  well  as  to  facilitate  tillage.  The  water 
is,  of  course,  held  on  each  level  strip  throughout  the  suc- 
cession of  terraces.    Where  gullies  have  already  formed, 
there  extension  may  usually  be  prevented  by  filling  with 
some  porous  material,  such  as  straw,  brush  or  stone,  which 
checks  the  flow  of  water  and  accumulates  the  sediment. 
Cross-embankments  are  also  useful.     When  these   are 
combined  with  the  growth  of  grass,  trees  or  other  plants, 
to  bind  the  soil  together  and  further  protect  it,  such 
land  can  frequently  be  reclaimed. 

361.  Erosion  by  wind. — Erosion  by  wind,  including 
the  drifting  of  sand,  may  be  checked  by  means  of: 
(1)  Windbreaks,  and  in  some  cases,  by  keeping  the  sur- 
face rough.  (2)  A  surface  covering  such  as  stone  or 
vegetation,  the  latter  to  bind  the  soil  together  and 
break  the  force  of  the  wind.  (3)  The  addition  of  organic 


CROP   ADAPTATION  497 

matter,  which  will  hold  the  soil  together  and  increase 
its  moisture  content,  which  latter  also  greatly  aids  the 
process.  (4)  In  fine-textured  soil,  such  as  silt  and  very 
fine  sand,  by  the  promotion  of  granulation  and  by  the 
avoidance  of  a  loose  fallow  surface  at  that  season  of 
the  year  when  wind  erosion  is  likely  to  be  serious. 

The  aggregate  of  soil  moved  from  tilled  fields  by 
erosion  of  these  two  types  is  large,  and  it  usually  con- 
cerns the  most  productive  portion.  The  encroachment 
of  sand-dunes  upon  valuable  land  is  often  a  serious 
menace.  Reforestation  and  the  planting  of  sand-binding 
grasses  are  the  chief  protective  measures  available. 

IV.     ADAPTATION    OF    CROPS    TO    SOIL 

It  is  a  matter  of  common  observation  that  all  crops 
do  not  grow  equally  well  upon  the  same  soil. 

362.  Philosophy  of  crop-adaptation.  —  Kach  plant  is 
adapted  to  make  its  best  growth  on  a  particular  soil 
and  under  a  particular  climate.  Any  departure  from 
these  ideal  conditions  results  in  changing  the  character 
of  the  plant  and  reduction  in  its  value.  This  peculiar 
adaptation  of  crop  to  soil  is  the  result  of  centuries  of 
natural  selection.  The  basis  of  all  the  tillage  operations 
which  have  for  their  object  the  modification  of  the  soil 
conditions  is  to  bring  the  soil  more  nearly  to  the  ideal 
condition  required  to  nourish  plants.  This  wide  differ- 
ence in  the  preferences  of  crops  is  well  known.  On  the 
other  hand,  there  are  hundreds  of  different  kinds  of 
soil, — that  is,  soils  which  normally  maintain  different 
conditions  for  growth.  Some  are  tine,  others  are  coarse, 

FF 


498 


THE   PRINCIPLES   OF  SOIL   MANAGEMENT 


some  are  deep,  others  are  shallow;  some  have  one  chemi- 
cal composition,  others  have  a  different  composition; 
some  are  dark,  others  are  light-colored;  some  are  wet, 
others  are  dry;  some  occur  under  one  climate,  others 


Fio.  154.     Lettuce  and  celery  growing  on  muck  soil.   Such  soil  usually  requires 
special  fertilization. 

under  another  climate.  It  is  the  combination  of  all 
these  factors  which  affect  plant  growth  that  gives  rise 
to  the  great  variety  of  soil  conditions.  The  great  variety 
of  plants  is  a  reflection  of  this  great  variety  in  the  con- 
ditions of  growth.  In  any  given  situation,  those  crops 
which  are  adapted  to  that  situation  persist  and  thrive. 


FACTORS  IN   CROP-ADAPTATION  499 

The  range  of  conditions  upon  which  a  particular  crop 
will  grow  is  limited.  It  is  wider  for  some  crops  than 
for  others.  Likewise,  the  range  of  crops  which  can  be 
grown  on  any  particular  soil  is  also  limited.  The  more 
extreme  the  soil  condition,  the  more  limited  is  this 
range  of  crop-adaptation.  It  is  the  soil  of  intermediate 
properties — texture,  organic  content,  drainage  and 
food  supply — which  is  adapted  to  the  greatest  variety 
of  crops.  In  the  largest  utilization  of  any  particular 
soil,  this  adaptation  of  crop  to  soil  must  be  made  use  of, 
as  well  as  modification  of  the  soil. 

363.  Factors  in  crop-adaptation. — The  determining 
factors  in  crop-adaptation  are  of  two  sorts:  (1)  The 
physiological  requirements  of  the  plant.  (2)  The  ca- 
pacity of  a  given  soil  and  climatic  condition  to  fulfil 
those  physiological  requirements. 

354.  Physiological  requirements  of  the  plant. — The 
physiological  requirements  of  the  plant  are  of  both  a 
physical  and  a  chemical  character. 

(1)  The  physical  requirements  relate  to  the  habits 
of  growth  of  the  plant,  particularly  the  type  of  its  root 
system  and  the  intensity  of  sunshine,  temperature  and 
wind  it   is  able  to  withstand.     Especially  important   is 
the  root  system.   Deep-  or  tap-rooted  plants  have  a  very 
different    feeding   ground    from  shallow-,  fibrous-rooted 
plants. 

(2)  The   chemical    requirements   relate   to   food   ele- 
ments necessary  to  growth,  and  especially  to  the  presence 
or  absence  of  accessory  substances  which  the  plant   is 
able  to  withstand.    For  example,  some  plants  will  not 
grow  in  a  soil  rich  in  lime;  others  require  this  condition. 


FACTORS   IN   CROP-ADAPTATION  501 

In  arid  soils  some  plants  are  able  to  withstand  alkali 
conditions  where  others  quickly  succumb.  There 
may  be  toxic  substances  in  the  soil  injurious  to  one 
plant  and  not  to  another.  These  may  arise  from  the 
growth  of  other  crops,  and  so  determine  the  plants  which 
may  be  associated  with  that  crop.  This  bears  on  crop- 
rotation. 

355.  Requirements  for  growth  supplied  by  the  soil.— 
The  internal  conditions  of  different  soils  may  be  very 
different.  On  a  puddled  clay  soil  saturated  with  water, 
only  a  few  plants  may  thrive.  On  a  dry  sandy  soil, 
only  certain  other  plants  can  secure  the  essentials  for 
growth.  On  a  very  shallow  soil,  shallow-rooted,  early- 
maturing  crops  may  be  grown  where  trees  would  utterly 
fail.  On  soils  subject  to  midsummer  drought,  early- 
maturing  crops  may  be  grown  where  late-maturing 
crops  would  fail.  Thus,  the  soil  conditions  are  the  arbiter 
in  the  selection  of  crops  to  be  produced.  The  distribu- 
tion of  different  crops  and  types  of  agriculture  is  a  re- 
flection of  this  adaptation.  Many  failures  result  from 
failure  to  recognize  these  relations. 

Full  knowledge  for  the  accurate  adaptation  of  crops 
to  soil,  or  soil  to  crops,  is  yet  to  be  gained.  Such  infor- 
mation is  often  not  to  be  derived  by  definite  experi- 
mentation. It  comes  of  long  experience.  Rut  many 
striking  examples  of  adaptation  are  known.  They  are 
governed  by  soil  conditions  broadly  considered,  rather 
than  by  any  single  factor.  One  of  the  most  general 
of  these  relations  is  the  adaptation  of  early  truck  crops 
to  light,  sandy  soil;  of  grass,  to  heavy  soil.  Certain 
varieties  of  apple  grow  to  their  highest  perfection  on 


•SM 


CROP-ROTATION  503 

certain  types  of  soil.  Cherries  and  peaches  succeed  best 
on  a  lighter  soil  than  apples  may  be  best  grown  on. 
Muck  soils  are  eminently  adapted  to  the  growth  of  celery, 
onions,  etc.  With  the  extension  of  careful  soil  and  crop 
surveys,  these  relations  are  becoming  better  known  and 
are  extending  from  groups  of  plants  to  species  and 
varieties  of  plants.  By  the  same  methods  our  informa- 
tion concerning  these  relations  must  be  extended  until 
the  production  of  crops  rests  upon  definite  knowledge 
of  the  plant  requirements  on  the  one  hand,  and  the  soil 
capacity  and  the  means  available  to  alter  the  soil 
environment  on  the  other  hand.  Really  intelligent  hus- 
bandry can  rest  only  upon  the  basis  of  exact  knowledge 
concerning  these  two  groups  of  facts  and  principles. 

V.     RELATION    OF    SOIL    PRODUCTIVENESS    TO    CROP- 
ROTATIONS 

At  an  early  time  in  the  development  of  agriculture, 
it  was  understood  that  a  succession  of  different  crops 
upon  any  piece  of  land  gave  better  returns  than  one 
crop  raised  continuously.  The  plan  of  changing  the 
crops  grown  each  year  thus  became  customary,  and 
the  universality  with  which  it  was  practiced  by  Euro- 
pean peoples  shows  that  its  value  must  have  been  dis- 
covered independently  in  many  communities,  as  ideas, 
particularly  agricultural  ones,  traveled  very  slowly  in 
the  middle  ages. 

In  (Ireat  Kritain  and  some  of  the  countries  of  Kurope, 
crop  rotations  have  been  most  systematically  and  effec- 
tively developed.  This  has  been  the  natural  result  of 


504         THE  PRINCIPLES   OF  SOIL  MANAGEMENT 

the  incentive  arising  from  diminishing  productiveness 
of  the  soil  consequent  upon  long-continued  cultivation, 
coupled  with  an  increasing  population.  Countries 
having  undepleted  and  uninfested  soil,  or  an  unpro- 
gressive  people,  have  done  little  with  crop-rotations. 

Another  condition  that  discourages  the  use  of  crop- 
rotation  is  the  suitability  of  a  region  to  the  production 
of  some  one  crop  of  outstanding  value,  combined,  per- 
haps, with  a  relatively  cheap  supply  of  fertilizing  ma- 
terial. The  abundant  use  of  fertilizers  may  postpone 
for  a  long  time  the  recourse  to  crop  rotations. 

356.  Principles  underlying  crop-rotation. — There  are 
many  benefits  to  be  derived  from  a  proper  rotation  of 
crops  that  are  not  directly  concerned  with  soil-produc- 
tiveness. The  practice  of  crop-rotation  must  depend 
upon  certain  principles  in  soil  management,  some 
of  the  most  prominent  of  which  are  mentioned  below, 
and  are  modified  by  climatic,  topographic,  geographic 
and  economic  features,  and  many  other  factors,  that 
cannot  be  treated  here. 

367.  Nutrients  removed  from  the  soil  by  different 
crops. — Some  crops  require  large  amounts  of  one 
fertilizing  constituent,  while  others  take  up  more  of 
another.  As  before  pointed  out  (see  page'  294),  cereal 
crops  are  able  to  utilize  the  potassium  and  phosphorus 
of  the  soil  to  a  considerable  degree  but  have  less  ability 
to  secure  nitrogen.  They  are,  therefore,  usually  much 
benefited  by  the  application  of  a  nitrogenous  manure  and 
leave  a  considerable  residue  in  the  soil.  A  number  of 
other  crops,  as,  for  instance,  beets  and  carrots,  can 
utilize  this  residual  nitrogen.  Grasses  remove  compara- 


PRINCIPLES   OF   CROP-ROTATION  505 

tively  little  phosphoric  acid.  Potatoes  remove  very 
large  amounts  of  potassium.  A  rotation  of  crops  is, 
therefore,  less  likely  to  cause  a  deficiency  of  some  one 
constituent  than  is  a  continuous  growth  of  one  crop, 
and  it  utilizes  more  completely  the  available  nutrients. 

358.  Root  systems  of  different  crops. — Some  crops 
have  roots  that  penetrate  deeply  into  the  subsoil,  while 
others  are  only  moderately  deeply  rooted,  and  others 
quite  shallow-rooted.    Among  the  deeply  rooted  plants 
are  alfalfa,  clover,  certain  of   the  root  crops,  and  some 
of  the  native  prairie  grasses.    Representing  those  having 
moderately  long  roots,  are  oats,  maize,  wheat,  meadow 
fescue,  grass,  etc.,  and  among  those  having  shallow  roots 
are  barley,  turnips  and  many  of  the  cultivated  grasses. 
As  plants  draw  their  nourishment  from  those  portions 
of  the  soil  into  which  their  roots  penetrate,  the  deeper 
soil  is  not  called  upon  to  provide  food  for  the  shallow- 
rooted  crops,   and  the  deep-rooted  crops  remove  rela- 
tively less  of  the  nutrients  from  the  surface  soil.     It 
therefore  happens  that  a  rotation  involving  the  growth 
of  deep- and    shallow-rooted    crops  effects,  by  utilizing 
a  larger  area  of  the  soil,  a  more  economical  utilization 
of  plant  nutrients  than  would  a  continuous  growth  of 
either  kind. 

359.  Some   crops  or  crop   treatments   prepare   food 
for  other  crops. — It  is  quite  evident  that  the  growth  of 
leguminous  crops,  even  when  not  plowed  under,  leave 
in  the  soil  an  accumulation  of  organic  nitrogen  trans- 
formed by  bacteria  from  atmospheric  nitrogen.     This, 
in  the  natural  course  of  decomposition  and  nitrification, 
becomes  available  to  cereal  or  other  crops  that  may  follow 


506          THE   PRINCIPLES   OF  SOIL   MANAGEMENT 

in  the  rotation.  The  presence  of  a  grass  crop  upon  the 
land  for  several  years  favors  the  action  of  non-symbiotic 
nitrogen-fixing  bacteria,  as  already  explained  (see  page 
429).  The  grass  crops  also  leave  a  very  considerable 
amount  of  organic  matter  in  the  soil,  which  by  its  gradual 
decomposition  contributes  both  directly  and  indirectly 
to  the  supply  of  available  nutrients.  As  the  organic 
matter  left  by  the  legumes  and  grasses  decomposes 
slowly,  these  crops  should  be  followed  by  a  coarse 
feeding  crop,  like  corn  or  potatoes,  and  one  which 
is  at  the  same  time  a  cultivated  crop,  as  are  these. 
Stirring  the  soil  at  intervals  during  the  summer  greatly 
facilitates  decomposition,  and  leaves  a  supply  of  easily 
available  food  for  more  delicate  feeders,  like  wheat  or 
barley,  that  may  follow  the  cultivated  crop.  The  intro- 
duction of  cultivated  crops  in  the  rotation  thus  serves 
to  prepare  food  for  the  non-cultivated  ones.  Although 
practical  difficulties  sometimes  make  it  impossible  to 
follow  the  cultivated  crops  with  winter  wheat,  the  prac- 
tice, where  proper  preparation  of  the  seed-bed  is  pos- 
sible, is  a  good  one. 

360.  Crops  differ  in  their  effect  upon  soil  structure. — 
Plants  must  be  included  among  the  factors  affecting 
the  arrangement  of  soil  particles.  The  result  of  practi- 
cally all  root  growth  is  to  improve  the  physical  condi- 
tions of  the  soil,  to  a  greater  or  less  degree.  In  general, 
crops  with  rather  shallow  and  very  fibrous  roots  are 
most  beneficial,  at  least  to  the  surface  soil.  Millet,  buck- 
wheat, barley,  and  to  a  less  extent  wheat,  leave  the  soil 
in  a  friable  condition.  It  is  upon  heavy  soils  that  this 
property  is  most  beneficially  exercised. 


508          THE   PRINCIPLES   OF   SOIL   MANAGEMENT 

Tap-rooted  plants,  and  others  with  few  surface  roots, 
do  not  exhibit  this  action.  Alfaifa  and  root  crops  are 
likely  to  leave  the  soil  quite  compact  as  compared 
with  the  crops  mentioned  above. 

The  effect  of  sod  is  generally  beneficial,  and  this  is 
one  of  the  reasons  for  using  a  grass  crop  in  a  rotation. 

361.  Certain  crops  check  certain  weeds. — By  rotating 
crops,  the  weeds  that  flourish  during  the  presence  of  one 
crop  upon  the  land  may  be  greatly  checked  by  succeed- 
ing crops.  Some  weeds  are  best  destroyed  by  smothering, 
for  which  purpose  small  grain  and  notably  corn  or  sor- 
ghum sown  for  fodder  are  effective.    Others  are  most 
injured  by  cultivation,  to  accomplish  which  the  hoed 
crops  are  needed;  while  others  can  best  be  checked  by 
the  presence  of  a  thick  sod  on  the  ground  for  a  number 
of  years.    In  the  warfare  against  weeds  that  must  be 
carried  on  wherever  crops  are  raised,  the  use  of  different 
crops  involving  different  methods  of  soil  treatment  is 
of  great  service. 

362.  Plant  diseases  and  insects  checked  by  removal 
of  hosts. — Many  plant  diseases  and  many  insects  spend 
their  resting  stages   and  larval  existence  in  the  soil. 
A  continuous  growth  of  any  one  crop  upon  the  soil 
favors  the  increase  of  these  species  by  providing  each 
year  the  particular  plant  upon  which  they  thrive.    A 
change  of  crops,  by  removing  the  host  plants,  causes 
the  destruction  of  many  diseases  and  insects  through 
their  inability  to  reach  their  host  plants.    A  long  rota- 
tion, such   as  is  frequently  used  in  Great   Britain,  is 
particularly  effective  in  eradicating  those  diseases  that 
persist  in  the  soil  for  a  number  of  years.    In  the  case  of 


REASONS   FOR   CROP-ROTATION  509 

diseases  that  affect  more  than  one  species  of  plant,  as 
does  the  beet  and  potato  scab,  there  is  need  for  special 
care  in  arranging  the  rotation.  Such  considerations 
may  frequently  make  it  desirable  to  change  the  plan  of 
a  rotation. 

Another  feature  of  the  relation  of  crop  rotation  to 
plant  diseases  is  that  the  more  thrifty  growth  obtainable 
under  rotation  assists  the  crop  to  withstand  many  dis- 
eases. 

363.  Loss  of  plant-food  from  unused  soil. — A  system 
of  crop-rotation   permits  a   more  constant   use  of  the 
land  than  is  possible  with   most   annual  crops.     As  a 
soil   bearing  no  crop  upon  it  always  loses  more  plant- 
food  than  one  bearing  a  crop,  it  is  thus  possible,  by  a 
well-chosen    rotation,    to   save    plant-food    that    would 
otherwise  be  lost. 

364.  Accumulation   of   toxic   substances. — That    the 
soil  frequently  contains  organic  substances  that  exert 
an  injurious  effect  upon  the  growth  of  certain  plants 
is  indicated  by  recent  experiments  and  was  surmised 
by  some  early  writers  upon  the  subject.     I)e  Candolle 
was   probably   the   first   to   advance   the   idea   in    ISiJ'J. 
He  suggested  that   at   least   some   plants  excrete  from 
their  roots  substances  that  are  injurious  to  themselves, 
although   harmless  or  even   beneficial   to  other  plants. 
This  he  considered  one  of  the  reasons  for  the  failure  of 
many  crops  to  succeed  when  grown  continuously  upon 
the  land,  while  that  same  soil  may  be  productive  under 
a  rotation  of  crops.     Liebig,  in   his  first    report   to  the 
British  Association  in   1S40,  made  a  similar  statement. 

Recently,  Pouget  and  Chonchak.  working  with  alfalfa 


510         THE  PRINCIPLES  OF  SOIL  MANAGEMENT 

soils,  have  reached  the  conclusion  that  alfalfa  plants 
excrete  a  toxic  substance  which,  gradually  accumu- 
lating in  the  soil,  injuriously  affects  the  growth  of 
alfalfa  plants.  Whitney,  Livingston,  Schreiner  and 
their  associates  conclude  that  certain  soils  contain  toxic 
substances  of  organic  nature  which  may  be  produced 
by  plant  roots,  or  possibly  by  certain  processes  of  de- 
composition of  organic  matter.  They  have  isolated 
from  soils  organic  compounds  that  are  poisonous  to 
plants. 

It  is  found,  for  instance,  that  cumarin,  which  is  a 
normal  constituent  of  sweet  clover  (Medicago  alba,  L 
and  M.,  officinalis,  P),  .may  be  obtained  from  certain 
soils,  and  that  it  is  toxic  to  wheat  seedlings, — from 
which  it  may  be  supposed  that  it  is  more  or  less  toxic 
to  other  plants.  Dihydroxystearic  acid  was  isolated 
from  certain  soils  by  Schreiner  and  Shorey,  who  found 
that  it  is  acid  to  litmus  and  decomposes  BaCO3  and 
CaC03,  forming  the  corresponding  salts.  The  extracts 
of  the  soil  containing  this  substance  were  toxic  to  wheat 
seedlings.  The  relation  of  soil  acidity  and  soil  toxicity 
is  thus  suggested. 

Working  with  different  media  in  which  wheat  and 
other  seedlings  were  grown,  it  was  shown  that,  where 
the  nutrient  solutions  were  very  dilute,  so  as  not  to 
enable  the  plant  to  overcome  the  effects  of  small  quan- 
tities of  toxic  matter,  the  wheat  plants  grew  much 
better  when  following  other  plants;  and  that,  in  spite 
of  a  renewal  of  the  supply  of  nutrients,  the  wheat  plants 
grew  less  well  when  one  crop  succeeded  another.  The 
cause  of  the  lessened  growth  was  attributed  to  the 


CROP   ROTATION   AND    TOXIC   MATERIALS          511 

excretion  from  the  plant  roots  of  substances  which, 
while  more  or  less  toxic  to  other  plants,  are  especially 
so  to  plants  of  the  same  species. 

Although  there  are  yet  many  phases  and  details  of 
this  subject  to  be  worked  out,  there  seems  to  be  some 
relation  between  the  presence  in  the  soil  of  organic 
substances  poisonous  to  plants  and  the  continuous 
growth  of  one  crop;  and  this  may  be  considered  to  be 
one  reason  for  the  benefit  derived  on  some  soils,  at  least, 
from  the  practice  of  crop-rotation. 


INDEX 


PAGE  PAOl 

Absolute  specific  gravity  jf  soil  ...   94        -I'.ulian  rocks 11,  14 

Absorption  by  the  soil 297        ^Eolian  soils,  deposition  of 60 

Causes  of ...  299  Composition  of 63 

Effect  of  adsorption .  .'101  Relation  to  loess 60 

Effect  of  aluminum  hydrate  .  .  .300  Aeration,  effect  on  availability  of 

Effect  of  calcium  carbonate 300    -  fertilisers 359 

Effect  of  ferric  hydrate 300  Effect  of  drainage  on 242 

Effect  of  humus 300  Effect  of  nitrification 416 

Effect  of  size  of  particle 299        Agencies  of  rock  decay 14 

Effect  of  zeolites 299  Plants  and  animals 28 

Influence  in  soil  analysis 277        Agricultural  classes  of  soil 74,  77 

Insolubility    of    absorbed    sub-  Air  (oxygen)  of  noil,  as  factor  in 

stances 299  plant  growth 1 

Occlusion 301  Circulation  of,  affects  soil  tem- 

Relation  to  drainage 302  perature 461 

Relation  to  productiveness 306  Air  of  .toil,  effect  of  on  percolation    167 

Time  required 297        Air  of  the  soil 432 

Absorption  of  nutrient  salts 286  Analyse!) 434 

Absorptive,  physical 102  Composition 434 

Absorption  properties  of  humus.  .  128  Carbon  dioxide  in 438 

Abundance  of  common  minerals    .      8  Effect  of  carbon  dioxide  produc- 

Acidity  of  soil,  effect  of  ammonium  tion  on 435 

sulfate 326    I         Effect  of  cropping  on 447 

Effect  of  lime 349  Effect  of  irrigation  on 447 

Effect  on  availability  of  fertiliz-  Effect  of  manures  on  . .                  444 

ere 360  Effect  of  organic  matter  on      .    433 

Effect   on   availability   of   phos-  Effect  of  roots  on  composition     437 

phorous 361  Effect  of  soil  moisture  on              433 

Effect  on  bacteria 360  Effect  of  structure    .                       432 

Effect    on    liberation    of    potas-  Effect  of  texture 432 

sium 360  Effect  of  tillage  on  .  .                       444 

In  relation  to  bacteria 401  Effect  of  undcnlrainage  on      .    445 

Acme  harrow,  efficiency  of 484  Escape  of  carlx>n  dioxide              436 

Adaptation  of  crops  to  soil,  exam-  Functions                                           437 

pies  of .r>03  Modifying    volume    and    move- 
Part  in  soil  management 465  ment  ...                                          443 

To  soil,  factors  in 499  Movements 439 

To  soil,  philosophy 497    ;         Movement   due    to   atmospheric 

To  soil,  lack  of  knowledge 501     :  pressure  .  .                                         440 

Adjustment  of  soil  moisture 170  Movement    due    to   gaseous   dlf- 

Adobe  soil,  relation  to  wind  forma-  fusion  .                                            439 

tion 62  Movement  due  to  temperature  441 

Adsorption  by  the  soil 301  Movement  due  to  water                440 

Effect  on  nitrates .    301  Movement  due  to  wind  .                443 

Relation  to  plant    nutrition          301    I        Oxidation  .  437 


GO 


514 


INDEX 


PAGE 

Algae 394 

Alinit 429 

Alkalies,  carbonate  of,  affect  struc- 
ture   118 

Alkali,  relation  to  irrigation 230 

Alkali  salts,  relation  of  drainage  to 

removal 247 

Alkali  soils 307 

Black  alkali 309 

Composition 309 

Correction  of  black  alkali 316 

Direct  effect  on  plants 312 

Effect  on  different  crops 313 

Effect  on  plants 312 

Indirect  effect  on  plants 313 

Reclamation 314 

Reclamation  by   growing   toler- 
ant plants 318 

Reclamation  by  leaching 318 

Reclamation  by  retarding  evap- 
oration   318 

Reclamation  by  underdrainage.315 

Relation  to  irrigation 314 

Relation  to  water  content 313 

Rise  of  alkali 314 

White  alkali 309 

Alkali  spots 314 

Reclamation 319 

Alluvial  soils 47 

Composition  of 52,  53 

Alternate,    cropping   in    semi-arid 

regions 196 

Aluminum  hydrate,  effect  on  ab- 
sorption  300 

Amount  of  water  in  soil 136 

Amount  of  water  used  per  irriga- 
tion  235 

Amendments 348 

Salt*  of  calcium 348 

Use  of,  part  in  soil  management .  465 

Ammonia,  conservation  in  manure  376 

Formation  in  farm  manures ....  376 

Ammonification 410 

Ammonium  sulf ate 326 

Composition  of 326 

Effect  on  acidity  of  soil 326 

Loss  from  soil 326 

Manufacture  of 326 

Amount  of  water  moved  by  soil .  .  .  185 
Amount  of  water  used  in  irrigation  .226 

Analysis  of  marine  clay  soils 50 

Arid  and  humid  soils 65 

Coastal  plain  soil* 49 


PAGE 

Analysis  of  common  minerals 6 

Cumulose  (muck)  soils 44 

Dust  and  loess  soils 63 

Earth's  crust 4 

Glacial  lake  soils 51 

Glacial  soils 57 

Humus  for  nitrogen 123 

Mechanical 70 

Residual  soils 32 

Soil  air 434 

Soil  separates 85,  86 

Animal,  age  of,  effect  upon  ma- 
nures   373 

Use  of,  effect  on  value  of  ma- 
nure  374 

Animals   and   plants,   agencies   in 

rock  decay 28 

Manures  produced  by  different .  368 
Animal  life,  effect  on  structure ...  1 18 
Apparent  specific  gravity  table  .  .   96 
Applying  water  in  irrigation,  meth- 
ods  228 

Aqueous  rocks    11,  12 

Areas  of  residual  soil,  in  America .  .   31 
Arid  and  humid  climates,  forma- 
tion of  mulches  in 198 

Arid  and  humid  soils 64 

Properties 79 

Arid  soils,  composition  of 65 

Organic  matter  in 125 

Arrangement  and  porosity 88 

Arrangement  of  soil  particles 88 

Ash  of  plants,  substances  found  in .  280 
Atmosphere,    as    agency    of    rock 

decay 16 

Composition  of 16 

Atmosphere  in  saturated  soil 1  .S9 

Atmospheric    pressure,    effect    on 

movement  of  soil  air 440 

See,  also,  Air. 

Attachments  for  plow 476 

Available  water  in  field  soils 157 

Availability  of  fertilizers 356 

Availability  of  phosphate  fertiliz- 
ers  339 

Soil  water 141 

Back  furrow,  meaning  of 475 

Bacteria 395 

Ammonification  produced  by.  .410 
Conditions  affecting  growth ....  399 
Decay    and    putrefaction    pro- 
duced by 408 


INDEX 


515 


PAGE 

Bacteria,  decomposition  of  cellu- 
lose  405 

Decomposition  of  mineral  matter403 
Decomposition  of  nitrogenous 

organic  matter 407 

Decomposition  of  non-nitrogen- 
ous organic  matter 404 

Decomposition  of  starch 405 

Distribution 396 

Effect  of  drainage  on 244 

Relation  to  organic  matter. 361,  401 
Relation  to  oxygen  in  the  soil.  .399 
Relation  to  temperature. 400,  450 

Relation  to  soil  acidity 401 

Relation  to  soil  moisture 399 

Soil,  functions 403 

Bacteria,  nitrification  produced  by  412 

Nitrobacter 413 

Nitro-bacteria 412 

Nitrogen  fixing 429 

Nitrosococcus 412 

Nitrosomonas 412 

Nitrous  ferments 413 

Numbers  in  the  soil 397 

Relation  to  nodules  on  roots.  .  .423 

Symbiotic  relation 423 

Y  andT  forms 426 

Bartlett,  determination  of  expan- 
sion of  rock 20 

Bases,  substitution  in  the  soil.  . .  .297 

Basic  slag 337 

Basins,  filter,  for  drainage 263 

Biological    processes,     relation    to 

soil  temperature 448 

Biological  enemies  in  soil,  as  fac- 
tors in  plant  growth 1 

Black  alkali 309 

Bobb,   sediment   carried   by   chief 

rivers 26 

Bone  phosphate 335 

Tankage 335 

Brands  of  fertiliiers 343 

Briggs,  amount  of  water  moved  by 

soils 1S7 

Figures  on  capillary  movement.  1SI 

Figures  on  surface  tension 160 

Buckingham,    amount    of     water 

moved  by  noil 1S8 

Diffusion  of  soil  moisture 189 

Calcium,  as  plant-food  element  .  .      3 
Calcium   carbonate,   effect  on  ab- 
sorption   300 


PAGE 

Calcium  carbonate,  reasons  for  de- 
termination   272 

Calcium  cyanamid .328 

Application  to  soil 330 

Composition  of 330 

Manufacture  of 329 

Calcium  nitrate 331 

Composition  of 332 

Manufacture  of 331 

Calcium  suits  as  amendments  . . .  .348 

Carbonate,  effect  on  soil 352 

Effect  on  plant  diseases 350 

Effect  on  soil  bacteria 348 

Effect  on  tilth 348 

Effect  on  toxic  substances 3£0 

Forms  used  on  soil 351 

Liberation  of  plant  food 349 

Oxide,  effect  on  soil 352 

Water-slaked 352 

Calculation,    of    apparent    specific 

gravity 96 

Number  of  particles  81 

Porosity ' 92 

Surface  area 83 

Cameron,  soil  moisture  and  physi- 
cal properties 156 

Campbell,  sub-surface  packer 212 

Capacity  of  soil  for  water 136 

Capillarity    affected    by    dampness 

of  particles 175 

Organic  matter 153 

Temperature  and  solution 173 

Capillary  efficiency  of  soil 185 

Capillary  efficiency  determined  by 

texture ISO 

Maximum 172 

Capillary    movement,    affected    by 

friction 1 72 

Affected  by  oily  substances .  .  .  .  183 

And  surface  tension 182 

Effect  of  structure 182 

Extent  anil  rate  of 1 73 

Horiiontally 184 

In  damp  and  dry  soil 180 

In  dry  soil 1 76 

Influenced  by  fprtilii-rs 182 

Measurement  of 1 75 

Modified  by  gravity 183 

Of  water 169 

Under  cloth  tent 215 

Capillary     water,     character     and 

amount  of 144 

Distribution  affected  by  gravity. 148 


516 


INDEX 


PAGE 

Capillary  water,  relation  to  texture  144 

Relation  to  structure 151 

Supplies  plants 142 

Carbonates,  of  alkalies,  affect  soil 

structure 118 

Carbon,  as  plant-food  element..  .  .     3 

Carbon,  in  humus 123 

Carbon  dioxide  in  relation  to  or- 
ganic matter 361 

Carbon  dioxide  in  soil  air 438 

Carbonate  of  calcium,  effect  on  soil  .352 

Case-hardening 101 

Caustic  lime,  effect  on  soil 352 

Cellulose,   decomposition   by  bac- 
teria  405 

Cementation  of  granules 106 

Cementing  materials 99 

Cereal  crops,  absorption  of  nutri- 
ents   294 

As  green  manures 387 

Chain  harrow,  use  of 486 

Chamberlin   and   Salisbury,   char- 
acter of  minerals 8 

Characteristics  of  minerals 8 

Characteristics  of  the  soil 2 

Checking  and  formation  of  gran- 
ules   106 

And  moisture  content 106 

Checking  of  soil 98 

Relation  to  evaporation 99 

Relation  to  drying 106 

Relation  to  weakness 106 

Chemical,  agencies  of  rock  decay.  .    14 
Chemical  analysis  of  soil,  complete 

solution 268 

Extraction  by  distilled  water.  .276 

Interpretation 270 

Manurial  needs 270 

Permanent  fertility 269 

Strong  hydrochloric  acid 269 

Use  of  carbon  dioxide  for 275 

Use  of  organic  acids 273 

Chemical  composition  of  soils.  ...   30 
Chemical  decay  of  rocks,  decom- 
position      14 

Chemical  effects  of  organic  matter.  131 
Chemical  precipitates,  rocks.  .  .11,  12 
Chemical  processes  in  soil,  depend- 
ent on  temperature 451 

Chemical    properties   of   arid   and 

humid  soil 64 

Chert,  influence  on  soil  formation .   40 
Chief  groups  of  soils 67 


PAGE 

Citric  acid,  use  in  soil  analysis ....  274 

Clark,  abundance  of  minerals ....     9 

Proportion  of  element  in  earth's 

crust 4 

Classes  of  manures 322 

Soil  textural      74,  76 

Number  of  particles  in 82 

Classification  of  rocks 10,  11 

Classification  of  soils 30 

Classification  of  soil  textural 70 

Clay  soil 74 

Clay  soil,  evaporation  in  checks.  .197 

Organic  matter  in 125 

Climate,  influence  on  percolation.  193 
Humid   conditions    requiring 

irrigation 226 

Relation  to  irrigation  practice .  .  224 

Clod-crushers,  types  of 489 

Clod,  relation  granule 91 

Cloth  tent,  effect  on  soil  moisture. 214 

Cohesiveness  of  soil 98 

Cold    and    heat,    agency    of    rock 

decay 18 

Cold  soils,  meaning  of 462 

Colloidal  clay  and  plasticity 97 

Colluvial  soils,  characters  of 45 

Colors  of  soil 101 

Color  of  soil,  affected  by  humus.  .  .  130 

Effect  on  temperature 456 

Commercial  fertilizers 322 

Constituents 324 

Function 322 

Compacting    soil,    importance    of 

drainage  in 240 

Composition  of  alluvial  soils .  .  .  52,  53 

Arid  and  humid  soils 65 

Atmosphere 16 

Effect  of  roots  on  composition. 437 

Escape  of  carbon  dioxide 436 

Glacial  soils 57 

Marine  soils  49,  50 

Residual  soils 32 

Hocks  and  residual  soils 32 

Soil  air 434 

Soil  air,  effect  of  carbon  dioxide 

production  on 435 

Soil-forming  minerals 6,  7 

Soil  separates 85 

Soil,  relation  to  rock 37 

Soil,  relation  to  texture 87 

Soils,  chemical 30 

Wind-formed  soil 63 

Composting  manure 380 


INDEX 


517 


PAGE 

Computation  of  value  of  fertilis- 
ers   345 

Conditions     affecting     growth     of 

bacteria 399 

Conditions  affecting  structure  ....  103 
Conditions  requiring  irrigation  .  .  .  .221 

Conductivity  of  heat  by  soil 459 

Conduction  of  heat, effect  on  tem- 
perature   452 

Constituents  of  soil 69 

Constituents  of  soil,  organic 119 

Control  of  erosion 494 

Control    of    soil    moisture,    means 

of 155,191 

Coulters,  for  plow,  use  of 476 

Covered  drains.    See  Underdrains. 

Cow  manure 369 

Crop  adaptation,  examples  of .  .  .  .503 

And  texture 80 

Factors  in 499 

I  .nek  of  knowledge 501 

Part  in  soil  management 465 

Philosophy  of 497 

Crop,  peculiarities  of,  in  water  re- 
quirement   224 

Crop  rotations 503 

Crop  rotation,  effect  on  soil  struc- 
ture   5O6 

Nutrients  renewed  by 5O4 

Place  of  manure  in 383 

Principles  of 504 

Relation  to  diseases  and  insects .  508 
Relation  to  loss  of  plant  food.  .  .509 
Relation  to  toxic  substances.  .  .  .509 

Relation  to  weed  growth 5O8 

Root  systems  of  different  crops  .505 
Some  crops  prepare  food  for 

ot  hers 505 

Crop  value  reduced  by  shrinkage  98 
Crop  yields,  effect  of  drainage  on  .  .  247 
Crop  yields  often  controlled  by  soil 

moisture 107 

Cropping  in  alternate  years 196 

Cropping,  effect  on  soil  air 447 

Crops,  absorptive  power,  for  nutri- 
ents   291 

Relation  of  to  use  of  irrigation 

water 235 

Relation  to  texture 79 

Cnimb  structure 91 

Cultivation,  best  time  for 478 

Cultivators,  types  of 478 

Effect  on  soil  . .  .  .478 


PAOB 

Cultivators,  harvester  type 489 

Seeder  types 484 

Cultures  for  soil  inoculation 428 

Cumulose  soils,  characteristics  of.   41 

Composition  of 44 

Occurrence  of 41 

Czapek,  experiments  on  solvent 
action  of  roots 288 

Dead  furrow,  meaning  of 475 

Decay,  agencies  of  rock 14 

Types  of 14 

Decay  of  organic  matter 120 

Promoted  by 132 

Retarded  by 132 

Rock,  type  of  and  composition 

of  soil 36 

Decay,  place  of  bacteria  in 408 

Decomposition,  type  of  rock  decay  14 
Decreasing  moisture  content  of 

soil 238 

Damp  and  dry  soil,  capillary 

movement 180 

Deep  plowing,  advantages  of  218,  221 
Deficiency  of  mineral  nutrients.  .  .280 

Deficiency  of  plant  nutrients 273 

Deflocculation.  effect  of  sodium 

nitrate 325 

Denitrification 420 

DeKnssi,  ex|>critnents  with  I'ttu- 

domonns  raiitnrolti 427 

Depth  best  for  mulches 207 

Depth  to  which  nitrification  ex- 
tends  418 

Deteriornt ion  of  fnnn  manure 375 

Detmer,  effect  of  humus  on  sand, 

water  rapacity 153 

Diameter  of  individual  pores  ...  94 

Dicyanamid 330 

Diffusion  of  moisture  vapor  in  soil  1*9 

Diffusion  of  soil  air 439 

Disc  harrows  nml  cultivator* 482 


Disc  plows,  efficiency  of  478 

Diseases  of  plants,  relation  to  crop 

rotation 5O8 

Disintegration,  by  atmospheric 

agency 18 

Type  of  rork  decay 14 

Distribution  of  bacteria 39<J 

D'Orbigny,  abundance  of  minerals  8 

Double  superphosphate* XA& 

Drainage,  by  surface  culture 263 

Availability  of  fertilisers .  .  .359 


518 


INDEX 


PAGE 

Drainage,  conditions  requiring  . .  .  238 

Depth  of  underdrains 254 

Effects  of  on  soil 239 

Frequency  of  underdrains 254 

Late  soils 243 

Methods  of 248 

Part  of  soil   management 465 

Principles  of 248 

Relation  to  absorption 302 

Soil   temperature 463 

Special  types  of 263 

See  Underdrains. 

Drainage  water,  composition 303 

Records  at  Rothamsted 303 

Dried  blood 333 

Dried  meat 334 

Dry  and  damp  soil,  capillary  move- 
ment in 180 

Dry  farming,  place  of  mulch  in.  .  .208 

Use  of  manure  in 383 

Dry  matter,  water  used  in  produc- 
tion   134 

Dry  soil,  capillary  movement  in.  .176 
Drying  of  soil,  influence  on  struc- 
ture  107 

Relation  to  checking 106 

Dust,  blankets 203 

Mulch,  management  of 205 

Storms 17 

Soil,  composition  of 63 

Volcanic 62 

Duty  of  water  in  irrigation 222 

Duty  of  water  in  irrigation,  factors 
affecting 224 

Early  soils,  causes  of 462 

Earthworms,  effect  on  structure.  .118 
Effects  of  organic  matter  on  soil. .  120 
Efficiency,  maximum  capillary.  .  .172 

Elements,   abundance  of 4 

Of  plant  food 3 

Proportion  lost  in  residual  soil 

formation    34,  36 

Engineering  in   irrigation 222 

Erosion,  agencies  causing 402 

By  water,  conditions  permitting  404 

Control  of 404 

Relation  of  drainage  to 247 

Evaporation 164 

Evaporation,  affected  by  checking    00 

Affected  by  winds 213 

At  Rothamsted 106 

Effect  of  on  soil  temperature.  .  .461 


PAOB 
Evaporation,  from  weeds  and  green 

manure 195 

From  soil,  wastes  water .  . 105 

Occurs  at  surface 196 

Prevented  by 199 

Prevented  by  special  treatment.  213 
Proportion    of    rainfall    lost    in 

United  States 106 

Relation  to  irrigation 197 

Relation  to  mulch  formation .  .  .  198 

Ridge  culture  to  prevent 216 

Excreta,  solid  of  manure 364 

Exhaustion  of  mineral  nutrients.  .285 

Exhaustion  of  nitrogen 328 

Exhaustion  of  plant  food 285 

Expansion,  of  minerals  and  rock 

by  heat    19,  20 

Soil  due  to  water 162 

Factors  in  plant  growth 1 

Factors  which  determine  soil  tem- 
perature   453 

Failyer  table,  composition  of  soil 

separates 86 

Fall  plowing 210 

Farm  manures 363 

Fermentations  of  manure 375 

Ferric  hydrate,  effect  on  absorp- 
tion  300 

Fertility  of  land,  relation  to  irri- 
gation   225 

Fertilizer,  brands 343 

Ammonium  sulf  ate 326 

Availability  of  phosphates 339 

Basic  slag 337 

Bone  phosphate 335 

Bone  tankage 335 

Calcium  cyanamid 328 

Calcium  nitrate 331 

Commercial 322 

Computation  of  value 345 

Constituents 324 

Containing  phosphoru* 334 

Cumulative  need  for 363 

Double  superphosphates 339 

Dried  blood 333 

Dried  meat 334 

Effect  of  soil  acidity  on  availa- 
bility  350 

Effect  of  soil  moisture  on  availa- 
bility  358 

Factor*  affecting  efficiency 355 

Function 322 


INDEX 


519 


PAOE 

Fertiliser,  Guano 333 

High  grade 343 

Hoof  meal 334 

In  relation  to  organic  matter  .  .  .361 

Insoluble  potassium 342 

Inspection 344 

Kainit 341 

Lime-nitrogen 330 

Low  grade 343 

Methods  of  applying 347 

Mineral  phosphates 335 

Mixing  on  the  farm 340 

Muriate  of  potash 341 

Nitrogen  lime 330 

Organic  nitrogen  in 332 

Part  in  soil  management 465 

Practice 342 

Reverted  phosphoric  acid 338 

Silvinit 341 

Sodium  nitrate 324 

Stassfurt  salts 340 

Steamed  bone 335 

Sulfate  of  potash 341 

Superphosphates 337 

Tankage 334 

Trade  value 344 

Used  for  their  nitrogen 324 

Used  for  their  potassium 340 

Wood  ashes 342 

Fertilizing,  for  cereal  crops 295 

Fruit  crops 296 

Grans  crops 295 

Leguminous  crops 296 

Root  crops 296 

Vegetables 296 

Field  soil,  surface  area 83 

Available  water  in 157 

Film  water 140 

Film  movement  of  water 169 

Horiiontally 184 

Film  water  ami  structure 106 

Film  moisture,  renewal  of  wastes 
water 198 

Flocculation,   affected   by   soluble 

salts 116 

Produced  by  carbonate  of  lime.3.52 

Produced  by  caustic  lime 352 

Relation  to  structure 116 

Flooding  in  irrigation  practice.  .  .  .229 
Crops  and  conditions  permitting  230 
Disadvantages  of 231 

Food,  in  soil,  aa  factor  in   plant 
growth 1.  2 


PAO« 

Food  supply  in  soil,  affected  by 

drainage 44 

Food  of  animal,  effect  on  value  of 

manure 375 

Force  of  frost  in  formation 20 

Formation  of  soils,  elements  lost 

in 34,36 

Forms  of  soil 141 

Friction,   effect  on   movement  of 

water 172 

Friction  determined  by  texture.  .  .  172 
Determines  capillary  efficiency .  180 
In  movement  of  soil  moisture, 

time  element 172 

In  moisture  movement,  relation 

to  mulch 182 

Retards  movement  of  moisture. 180 
Frost,  force  of  in  disintegration.  . .  20 
Fruits,  absorption  of  nutrients.  .  .296 

Functions  of  manures 384 

Functions  of  soil 1 

Soil  air 437 

Soil  bacteria 403 

Water  in  soil  and  plant 133 

Fungi,  large,  effect  on  the  soil.  .  .  .390 

Microscopic,  in  the  soil 392 

Furrow  back,  meaning  of 475 

Dead,  meaning  of 475 

Irrigation,  conditions  and  crops 

permitting 231 

Irrigation,  disadvantage  of 231 

Irrigation,  principles  of 231 

Proper  width  and  depth  of 472 

Gallagher,   expansion    of    soil    by 

water 162 

On  soil  shrinkage 98 

Soil  moisture  and  physical  prop- 
erties   150 

Geological  classification  of  soils.  .  .   30 

Geology,  not   soils 2 

Relation  to  soil  study 2 

Georgeson,  on  effect  of  manure  on 

soil  temperature 464 

Germination,  effect  of  soil  tempera- 
ture on 448 

Gilbert,  and  Laws,  water  used  by 

plants 134 

Glacial  ice,  agency  in  rock  decay.  .    27 

Influence  on  topography 28 

Glacial  soils,  character  of 54 

Chemical  character  of M 

Composition  of 57 


520 


INDEX 


-PAGE 

Glacial  soils,  modified  by  water  . .   59 

Occurrence  of 56 

Physical  character  of 57 

Relation  to  underlying  rocks  .  55,  60 

Grain,  separate,  structure 89 

Granular  structure 91 

Granulation,  relation  to  texture. . .  92 

Due  to  ice  crystals 108 

Relation  to  soil  moisture 156 

Granules,  cementation  of 106 

Relation  to  checking 106 

Grass  crops,  absorption  of  nutri- 
ents   295 

Protection  to  surface 113 

Gravitational  water 160 

Capacity 162 

Injurious  to  crops 163 

Movement  of  water 165 

Relation  to  porosity 161 

Relation  to  texture 161 

Water,  affected  by  percolation .  164 
Gravity,  affects  capillary  distribu- 
tion  148 

As  agency  of  soil  transportation .   45 

Green  manures 384 

Green  manures,  and  humus  supply. 132 

Cereal  crops 387 

Leguminous  crops 385 

May  be  injurious 195 

Ground  limestone,  effect  on  soil.  .  .353 
Growth  of  plants,  influence  of  tem- 
perature on 449 

Growth,  requirements  of  plants  for.      1 

Guano,  composition 333 

Gyp»um,  effect  on  soil  354 

Hall,  effect  of  sodium  nitrate  on 

structure 118 

Hardpans,  broken  up  by  subsoil- 
ing 219 

Hardpan  soil,  objections  to 475 

Hard     surface    soil,    repels     rain 

water 216 

Harrow,  as  cultivator 478 

Scotch  chain 486 

Types  of 482 

Use  in  mulching  land 209 

Heat  and  cold,  agency  of  rock  de- 
cay      18 

Frost  action  on  rock 20 

Mechanical  action  of 19 

Heat,  increases  solvent  action.  ...    19 
Heat  of  the  soil,  functions  of 448 


MM 

Heat  of  soil  as  factor  in  plant 
growth 1 

See,  alto.  Temperature. 
Heat,  specific,  of  soil  and  water. .  .455 
Heaving,  relation  of  drainage  to  ...  245 
Hellriegel,  best  soil  moisture  con- 
tent  156 

Water  used  by  plants 134 

Henneberg  and  Stohmann,  experi- 
ments with  soil  absorption .  .  .  297 

High  grade  fertilizers 343 

Hilgard,  nitrogen  in  humus 123 

Volume  weight  humus 128 

Hillside  plow,  advantages  of 475 

Hoof  meal 334 

Horizontal  movement  of  water.  .  .172 
Horizontal  movement  under  field 

conditions 184 

Horse  manure 368 

Hosmer  and  Whitney,  soil  moisture!57 

Humates 127 

Humid  and  arid  soils 64 

Conditions,  efficiency  of  mulch .  198 

Properties 79 

Humid   climate,   requirements   of 

irrigation  in 226 

Organic  matter  in 125 

Regions,    difficulty    of    keeping 

mulch 204 

Soils,  composition  of 65 

Humus,  defined 121 

Humus,  carbon  in 123 

Affects  water  capacity 163 

Effect  on  absorption 300 

Effect  on  color  and  temperature 

of  toil 457 

Effect  on  structure 113 

Nitrogen  in 123 

Plasticity 129 

Some  properties  of 115 

Volume  change 128 

Hunt,  growth  of  com  crop 135 

Hydration,  defined 21 

Hydration,  example  of 21 

Hydrogen,  as  plant  food  element.     3 
Hygroscopic  water,  character  and 

amount 143 

Relation  to  texture 144 

Use  of 194 

Ice  as  agency  of  rock  decay 27 

Ice  cryitals  form  lines  of  weakness  .108 
Relation  to  granulation 108 


INDEX 


521 


PAGE 

Ice-formed  soils,  character  of.  ...   54 

Igneous  rocks 11,  12 

Implements  for  creating  mulch  .  .207 

Implements  of  tillage,  group 460 

Implements  used  to  increase  water 

capacity 217 

Inoculation,  part  in  soil  manage- 
ment   465 

Of  soil  by  means  of  cultures.  .  .  .428 

Of  soil  for  legumes 425,  427 

Inorganic  constituents  of  soil.  ...    69 

Insects,  effect  on  the  soil 390 

Relation  to  crop  rotation 508 

Insolubility  of  the  soil 267 

Inspection  of  fertilisers 344 

Iron  as  coloring  material 101 

As  plant  food  element 3 

Oxides  as  cementing  material.  .  100 
Irrigation,    amount    of    water    to 

use 226,  235 

Conditions  requiring 221 

Duty  of  water  in 222 

Effect  of  on  alkali 236 

Effect  on  soil  air 447 

Engineering     and      agricultural 

phases 222 

Evaporation  affects  practice.  .  .  197 

Kind  of  soil  most  suited  to 237 

Land    in   United  States   requir- 

ing 223 

Of  tobacco  in  Florida 237 

Part  of  soil  management 465 

Relation  of  climate  to  practice 

of 224 

Relation  of  tillage  to  practice.  .225 

Relation  to  alkali 314 

Skill  required  in 228 

Time  to  apply  water  in 224 

Units  for  measuring  water  in  ..  .227 
When  to  apply  water 197 

Jointer,  for  plow,  use  of 476 

Kainit 3 

Kaolinite,  character  of 

King,  amount  of  water  moved  by 

soils 186 

Effect  of  drainage  on  soil   tem- 
perature .... 

Effect  of  plow  on  soil  structure  111 
Effect  of  trees  on  evaporation  .214 
Experience  with  dust  mulrh  .207 
Experiment  in  spring  plowing  211 


PAOE 

King,  extent  of  soil  moisture  ad- 
justment    .  . 184 

Figures  on  effect  of  drainage  on 

temperature .  .  463 

Figures  on  effect  of  suhsoiling.    219 
Figures  on  percolation  ....  166 

Observations  on  action  of  plow  -169 

On  temperature  of  soil 459 

Skill  required  in  irrigation 228 

Small    rainfall    increases   loss  of 

soil  water 198 

Suggestions  on  structure 103 

Water  used  by  plants 134 

Kunie,  experiments     on     solvent 

action  of  roots 289 

Lake  or  lacustrine  soils 47 

I^nd    in    United   States    requiring 

irrigation 223 

Lang,   figures  on   specific   heat    of 

soil 456 

Late  soil,  causes  of 462 

Relation  of  drainage  to  .  .243 

Ijjwes  and  (iilbert,  water  used  by 

plants 134 

I/eaching  of  manure  .    377 

( >rganic  matter  ....  .127 

Leaf  mold 121 

I-egumcs,  bacteria  on  roots.  .  .        423 
I/eguminoua   crops   for   green   ma- 
nure        385 

Absorption  of  nutrients  296 

legumes,  inoculation  of  soil  for       427 

Legumes,  inoculation  of  coil  for       425 

I/evees  used  in  drainage  263 

I/evel  culture,  generally  best  216 

Lime,  carbonate  effect  on  soil          352 

As  cementing  material  100 

Lime,  caustic  effect  on  soil  352 

Kffect   on   hn.iiu"  127 

Kffcct  on  plant  diseases  35O 

Kffect  on  soil  bacteria  348 

F.ffect  on  structure  116 

F.ffect  on  tilth  . .  '  - 

Kffect  on  toxic  substance*  350 

Forms  of.  in  relation  lout  picture  1 17 

(tround  limestone,  effect  on  noil   353 

Liberation  of  plant  food  34» 

Lime-nit  mgen  33O 

Lime,  relation  to  magnesium  .I.V) 

As  a  soil  amendment  34H 

Limestone  soils  . 

Lister.  <onditions  where  useful      .  .485 


522 


INDEX 


PAGE 

Loess,  composition  of 63 

Physical  characters  of 61 

Relation  to  wind  formation ....   60 
Soil,  extent  of 61 

Loose  top  soil  absorbs  rainfall ....  217 

Loss  of  nitrates  from  soil 418 

Water  from  soil 164 

Loughridge,     table,     composition 
soil  separates 85 

Low-grade  fertilizers 343 

Macro-organisms  of  the  soil 388 

Maintenance  of  organic  matter. .  .131 
Magnesium,  as  plant  food  element .     3 

Magnesium,  relation  to  lime 350 

Management  of  dust  mulch 205 

External  factors  in 463 

Mulches,  summarized 210 

Soil  and  texture 80 

Soil  moisture,  highest  type  of.  .238 

Manures 319 

Manure,  early  ideas  of  function.  .  .320 
As  affected  by  food  of  animal.  .  .373 
As  affected  by  use  of  animal .  . .  .374 
As  affected  by  age  of  animal.  . .  .373 

Composting 380 

Conservation  of  ammonia 376 

Deterioration  of 375 

Different  classes 322 

Effect  on  volume  and  movement 

of  soil  air 444 

Factors  affecting  value 373 

Farm,  formation  of  ammonia.  .  .376 

Farm,  loss  of  free  nitrogen 377 

Fermentations  of 375 

For  cereal  crops 295,  387 

For  grass  crops 295 

For  leguminous  crops 296 

For  root  crops 296 

For  vegetables 296 

From  animals,  relative  values.  .371 

From  poultry 372 

From  sheep 371 

From  swine 370 

From  the  cow 369 

From  the  farm 363 

From  the  horse 368 

Function  of 384 

Green 384 

Green,  leguminous  crops 385 

In  maintaining  humus 132 

Leaching 377 

Litter..  367 


PAGE 
Manure,  methods  of  handling  .  . .  .379 

Nutrient  function 320 

Place  in  crop  rotation 383 

Produced  by  different  animals .  .  368 
Relation  to  soil  moisture  supply  22f 

Sawdust 368 

Solid  excreta 364 

Straw  as  litter 367 

Urine 366 

Use  of  in  dry  farming 383 

Mass  of  soil 2 

Marine  soils 46 

Marine  soils,  composition  of 49 

Marl,    associated    with    cumulose 

soil 43 

Maximum  capillary  efficiency  ....  172 

Moisture  content 155 

Water  capacity 161 

Materials,  coloring  in  soil 101 

Mead,  number  of  methods  of  apply- 
ing water 229 

Means  of  decreasing  soil  moisture .  238 

Methods  of  applying  fertilizers.  .  .  .347 

Applying  water  in  irrigation ....  229 

Handling  manure 379 

Mechanism  of  the  soil 2 

Mechanical  action  of  water  in  rock 

decay 24 

Analysis  defined 70 

Decay,  of  rocks,  disintegration .  .    14 
Support  by  soil  as  factor  in  plant 

growth 1 

Meeker  harrow,  efficiency  of 484 

Merrill,    composition    of    residual 

soils 33 

Example  of  atmospheric  disin- 
tegration      IS 

Examples  of  expansion  of  rock .  .   20 
Method  of  calculating  loss  of 

rocks  in  decay 31 

Table,  composition  soil  separates  84 

Metamorphic  rocks 11,  14 

Micro-organisms,  plant 392 

Micro-organisms  in  the  soil 391 

Miners'  inch,  defined 228 

Minerals,  characteristics  of 8 

Absorbed  by  crops 291 

Amounts  removed  by  crops.  . .  .281 
Composition   of   important   soil 

forming 5 

Constituents  of  soil,  absorption 

by  roots 279 

Deficiency 273,  280 


INDEX 


523 


PAGE 

Minerals,  exhaustion  of 285 

Groups  of 5 

Important  soil  forming 4 

Matter,  decomposition  by  bac- 
teria  403 

Nutrients,    amounts    contained 

in  soils 282 

Phosphates,  composition  .  ..335,  336 

Relative  abundance  of 8 

Table  of  chemical  and  physical 

properties 6,  7 

Minimum    capillary    content    and 

wilting 182 

Minimum  moisture  content 155 

Mixing  fertilizers  on  the  farm.  .  .  .346 
Moisture  in  soil,  as  factor  in  plant 

growth 1 

.  Affected  by  manure* 221 

As  related  to  soil  air 433 

-  Capacity  of  soil  effect  of  texture 

and  structure 216 

Capillary  form 144 

Content  and  structure 105 

Control  of 190 

Critical  content 155 

Decreasing  I"--  of 191 

Diffusion  of  vapor  through  soil.  190 
Effect  of  early  spring  plowing.  .211 

.  Effect  of  salt 355 

.Effect  of    structure,    on    move- 
ment   182 

Effect  on  porosity 162 

Effect  of  tent  on 214 

Effect  on  heat  conductivity.  .  .  .460 

Effect  on  soil  temperature 461 

Forms  and  availability 141 

Gravitational  form 160 

Hygroscopic  form 143 

Increased  loss  through  rainfall .  198 

Maximum  content 155 

Means  of  decreasing 238 

Minimum  content 155 

Of  soil,  adjustment  of 170 

Prevention  of  percolation 191 

Relation  to  bacteria 399 

Should  be  stored  deep 198 

Special    treatment    to    prevent 

evaporation 213 

Used  by  different  crops 185 

Use  of  hydrostatic  form 194 

See  Water. 

Modification  of  structure 104 

Soil  temperature  means 463 


PAGE 
Modification  of  structure,  texture.  87 

Molds,  slime 394 

Moldboard,    relation   of   shape   to 

character  of  soil 472 

Movement  of  plant  roots  in  soil ...  174 

Soil  air 439 

Soil  air,  methods  for  modifying.   443 
Soil  moisture,  affected  by  damp- 
ness   175 

Soil  moisture,  amount  of 185 

Soil  moisture,  due  to  texture.  .  .  172 
Soil  moisture  supplemented  by 

roots 1 74 

Soil  moisture,  thermal 189 

Water,  affected  by  friction  ....  127 

Water  affected  by  solution 173 

Water,  affected  by  structure.  .  .182 
Water  by  soil,  lack  of  data  on .  .  1S5 

Water  horizontally 172 

Water  in  soil 165 

Muck 355 

Muck,  defined 121 

Ammonia  ext  ract ,  effect  on  struc- 
ture   115 

Composition 355 

Crude,  effect  on  structure.  .  115,  355 

Organic  matter  in 125 

Effect  on  soil  moisture 355 

Formation  of 41 

Quantity  added  to  soil 355 

Mulches,  principles  of 199 

Depend  on  rapid  evaporation     .  198 
Depend  upon  preventing   diffu- 
sion   189 

Depth  of 207 

Effectiveness      determined      by 

friction 182 

Effect  on  structure 119 

Example  of  effect,  Cornell  Uni- 
versity  207 

Equaliie  soil  moisture 198 

Implements  for  creating 207 

Importance  of  texture 205 

Kinds  of 199 

Management  of  dust 205 

Management  of,  summarised  .  .  .210 

May  be  compact  soil 205 

Most  effective  in  arid  regions    .  .204 
Natural   formation   in   arid   and 

humid  climates 198 

Relation  to  irrigation  practice.  .236 

To  prevent  wind  erosion 496 

When  may  be  formed  by  roller    .487 


524 


INDEX 


PAGE 

Mulching,  grain  fields 208 

Plow  land 209 

Muriate  of  potash 341 

Nature,  method  of  tillage 18 

Nematodes  in  the  soil 391 

Nitrogen,  as  plant  food  element.  ..     3 

Fixation  by  symbiosis 423 

Fixation  without  symbiosis.  .  .  .429 

Fixing  organisms 429 

Per  cent  in  humus 123 

Transfer  from  nodules  to  plant .  425 

Nitrogen  lime 330 

Free,  loss  from  manures 377 

Nitrification 412 

Nitrification,    depth    to    which    it 

extends 418 

Effect  of  aeration  on 416 

Effect  of  organic  matter  on.  .  .  .413 

Effect  of  sod  on 417 

Nitrates  in  soil,  affected  by  drain- 
age  244 

Loss  from  soil 418 

Of  soda,  effect  on  structure.  ...  118 

Reduction  to  ammonia 420 

Reduction  to  nitrites 420 

Nitragin 428 

Nitrites,  reduction  to  free  nitrogen420 

Nitro-bacteria 412 

Nitrobacter 413 

Nitrosococcus 412 

Nitrosomonas 412 

Nitrous  ferments 413 

Nobbe,  on  germination  and  tem- 
perature   449 

Nodules 423 

Nodules,    transfer   of   nitrogen   to 

plant 425 

Number  of  bacteria  in  soil 397 

Particles,  calculation 81 

Particles  in  soil 81 

Nutrient  salts,  absorption  of 286 

Selective  absorption 286 

Occlusion,  relation  to  soil  absorpt'nSOl 
Oily  substances   on   soil   particles 

affects  capillarity 183 

Open  drains,  advantages  and  dis- 
advantages of 250 

Construction  of 250 

Conditions  permitting 248 

Organic  acids,  use  for  soil  analysis .  273 
Organic  constituent  of  soil 1 19 


PAOB 
Organic  matter,  amount  in  soil.  .  .124 

Absorptive  properties 128 

Affects  water  capacity 153,  361 

And  raw  phosphates 361 

As  coloring  material 101 

Composition 121 

Derivation  of 120 

Effect  of  decay  on  temperature 

452,464 

Effects  of  on  soil 129 

Effect  on  nitrification 413 

Effect  on  structure 1 13,  361 

In  relation  to  bacteria 361,  401 

In  relation  to  carbon  dioxide  . .  .361 

In  relation  to  fertilizers 361 

In  relation  to  soil  air 433 

Loss  by  leaching 127 

Maintenance  of  in  soil 131 

Nitrogenous,  decomposition  by 

bacteria 407 

Non-nitrogenous,  decomposition 

by  bacteria 404 

Properties  of 126 

Sources  of 120 

Used  to  prevent  erosion 497 

Volume  change 128 

Organic  nitrogen  in  fertilizers.  . .  .332 

Organic  soils 43 

Organisms  in  the  soil 388 

In  soil,  effect  of  drainage  on ....  244 

Macro-,  in  the  soil 388 

Micro-,  in  the  soil 391 

Of  soil,  importance  of  tempera- 
ture for 450 

Osmotic  equilibrium  in  plants ....  287 
Outlet  of  underdrains,  importance 

of 261 

Oxygen,  as  plant  food  element.  ...     3 

Relation  to  soil  bacteria 399 

Oxidation,  a  function  of  soil  air.  .  .437 

Packers  and  crushers 486 

Packing  of  subsoil,  benefits  of .  . .  .212 
Parkes,  effect  of  drainage  on  soil 

temperature 243 

Particles  of  soil,  constitution  of . . .   69 

Number  of  in  soil 80 

Number  of  calculation 81 

Of  soil,  mineral  character 70 

Of  soil,  shapes 70 

Surface  area  of 82 

Peat,  defined 121 

Formation  of 41 


INDEX 


525 


PACE 

Percolation 164 

Percolation  of  water,  factors  affect- 
ing  167 

Direction  of 168 

King's  figures  on 166 

Prevention  of 191 

Relation  to  climate 193 

Rothamsted  figures  on 192 

Phosphate  fertilizer* 334 

Ph>sical  agencies  of  rock  decay  . .    14 

Absorption 102 

Character  of  soil,  relation  to  irri- 
gation   224 

Chief  groups  of  soil 66 

Effects  of  organic  matter 129 

Of  arid  and  humid  soil 64,  79 

Of  soil 68 

Processes  in  soil,  relative  to  tem- 
perature   431 

Properties  of  soil-forming  miner- 
als   6,  7 

Properties  of  soil  most  suited  to 

irrigation 237 

Piedmont  soils 40 

Planker,  drag  or  float,  efficiency  of  .489 

Plants  amount  of  water  used  by  .  .  133 

And   animals,   agencies   in    rock 

decay 28 

Plant  food,  elements  of 3,  291 

As  factor  in  growth 1 

Derived  from  rocka 3 

Derived  from  air  and  water.  ...      3 

Diseases,  effect  of  lime 350 

Elements,  abundance  of 4 

Elements  in  soil,  abundance  .68,  282 

Exhaustion  of 285 

Groups  of 3 

In  the  soil,  amounts  removed  by 

crops 281 

Relation  to  crop  rotations 509 

Plant  growth,  factors  in 1 

Influence  of  soil  temperature  on  449 
Plants,    injured    by    gravitational 

water 163 

Micro-organisms  in  the  noil 392 

Physiological     requirements     in 

adaptation 499 

Requirements  for  growth 1 

Result  of  the  inherent  capacity 

of  seed 1 

Result  of  environment 1 

Roots,  effect  on  the  soil 391 

Roots,  effect  on  soil  itructure  .  .  .113 


PAGE 

Plants,  roots,    importance    of   ad- 
vance through  soil 174 

Roots,    strike    deep    in  drained 

soil 245 

Plasticity,  cause  of 97 

Of  humus 129 

Properties  of 97 

Plows,  types  of 471 

Attachments  for  uses 476 

Efficiency  of  depends  on 470 

Effect  on  soil  structure Ill 

Hillside 475 

Mode  of  action 469 

Moldboard,  adaptation  to  soil.  .471 
Relation  of  use  to  soil  moisture. 470 

Subsoil 477 

Plowing  deep,  advantages  of 218 

Relation  to  humus  supply  .  .  .  .218 

Plow-land,  mulching 209 

Plow-sole,  character  of 474 

Pore  space  in  soil 93 

94 
21 
162 
92 
92 
340 
202 
4S6 


Diameter  of  individual 

Porosity  of  rocks 

Affected  by  moisture 

Calculation  of 

Of  soil 

Potassium  fertilizers 

Potato  growing,  straw  mulches  in 

Harvester,  as  cultivator 

Potts,  figures  on  heat  conductivity 

of  soil 4.M) 

Poult  ry  manure 372 

Practices  used  in  soil  management  .465 
Practices  in  soil  inanaKcinrnt,  pri- 
mary and  secondary  functions 

of 465 

Pressure  of  air,  effect  on  percola- 
tion   168 

Primary  minerals 5 

Process  of  atmospheric  rock  decay  .    16 
Productiveness  of  soil  de|MMids  mi      36 

In  relation  to  crop  rotation 

Properties  of  soil 

Properties  of  soil  separates    . 
Puddled  structure.  . 


Puddled  soil,  changed  by  lime.  . 

Danger  frurn   subsoiling    . 

Movement  of  water  in 

Pulverisation,  action  of  plow  in 
Putrefaction 


Oil 
so 

90 
117 

478 
IS2 
469 
408 


Quinke  estimates  molecular  attrac- 
tion ..  146 


526 


INDEX 


PAGE 

Rainfall,  United  States,  map  of  .  .  137 

Effect  on  structure H9 

Relation  to  irrigation  practice  . .  222 
When  small,  may  cause  loss  of 

water 198 

Rains,  gentle,  best 216 

Raw  phosphates  and  organic  mat- 
ter  361 

Reclamation  of  alkali  soil  by  drain- 
age   247 

Reduction  of  nitrates  to  nitrites.  .420 

Nitrates  to  ammonia 420 

Nitrites  to  free  nitrogen   420 

Relation  of  lime  to  magnesium ....  350 

Residual  soils 31 

Characteristics  of  section 40 

Proportionate  loss  of   elements 

in  formation,  table 34 

Texture  of 41 

Reverted  phosphoric  acid 338 

Ridge  culture, effect  on  evaporation  216 

Ries,  on  plasticity 97 

Rise  of  alkali 314 

Roberts,  plow  sole 474 

Rodents  effect  on  the  soil 388 

Rocks,  as  source  of  soil 2 

Aggregates  of  minerals 10 

And  its  products 2 

Aqueous 11, 12 

Classification  of 10 

Decay,  agencies  of 14 

Decay,  atmospheric 16 

Decay,  by  solution 22 

Decay,  ice  as  agency 27 

Decay,  type  of  and  composition 

of  soil 36 

Decay,  type  of   and  texture  of 

soil 36 

Expansion  due  to  heat 20 

Igneous 11, 12 

Important  soil-forming 9 

Porosity  of 21 

Roller,  conditions  where  effective  .487 

And  packers,  types  of 486 

Effect  on  soil  temperature 459 

Relation  to  percolation 194 

When  used  to  mulch  soil 487 

Roots,  absorbing  system 292 

Absorption  of  mineral  matter. .  .  277 
As  related  to  crop  rotations.  .  .  .505 
Crops,  absorption  of  nutrients .  .  296 
Conditions   where   enter   drains 
253,261 


Roots,  effect  on  composition  of  soil 

air 437 

Excretion  of  acids 290 

Hairs,  relation  to  soil  particles.  .287 
Of  plants,  strike  deep  in  drained 

soil 245 

Osmotic  activity 292 

Osmotic  equilibrium  in 287 

Plants,  effect  on  the  soil 391,  113 

Rotation  of  crops 503 

Rotation  of  crops,  principles  of.  .  .504 

Effect  on  soil  structure 506 

Nutrients  removed  by 504 

Part  in  soil  management 465 

Place  of  manure  in 383 

Relation    to    diseases    and    in- 
sects    508 

Relation  to  loss  of  plant  food  . .  .  509 
Relation  to  toxic  substances.  .  .509 

Relation  to  weed  growth 508 

Root  systems  of  different  crops .  505 
Some    crops    prepare    food    for 
others 505 

Rothamsted,  figures  on  evapora- 
tion  196 

Tables  of  percolations 192 

Rubbish,  covering  by  plow 476 

Salisbury,  and  Chamberlin,  char- 
acter of  minerals 8 

Sachs,  experiments  on  solvent  ac- 
tion of  roots 288 

Salt,  effect  on  soil 354 

Sand-dunes,  damage  by 497 

Naturally  mulched 206 

Sandstone  soils 39 

Sandy  soil 74 

Sandy  soil,  organic  matter  in 12 

Sanitary  conditions,  effect  of  drain- 
age on 247 

Saturation,  total  water  for 161 

And  gravitational  water 

Wave  of  in  soil,  useful 194 

Sawdust  as  litter  in  manure 368 

Schubler,  figures  on  effect  of  color 

on  temperature 457 

On  soil  shrinkage 98 

Season,    length    of    increased    by 

drainage 243 

Secondary  minerals 5 

Sedimentary  rocks 11,  12 

Sedentary  soils 30 

Sedentary  soils,  division*  of 31 


INDEX 


527 


PAGE 

Sediment,  in  water  of  chief  rivers .  .    26 

Seeder  types  of  cultivators 485 

Selective    absorption    of    nutrient 

salts 286 

Semi-arid  soils,  composition  of  ....   65 

Separate  grain  structure 89 

Separates,  properties  of  soil 80 

Series,  soil,  defined 78 

Sewage  irrigation 222 

Shale  soils 39 

Shapes  of  soil  particles 70 

Sheep  manure 371 

Silica  as  cementing  material 101 

Silting  up  of  tile  drains 258 

Silvinit 341 

Sue  of  particle,  influence  on  soil 

absorption 299 

Slime  molds 394 

Slope  of  soil,  influence  on  tempera- 
ture   458 

Shrinkage  of  soil 98 

Sod,  effect  on  nitrification 417 

Sodium  nitrate 324 

Sodium  nitrate,  composition  of.  .  .325 

Deflocculatingaction 326 

Effect  on  structure 118 

Effect  on  plant  growth 325 

Preparation 325 

"Soil"  denned 68 

Adaptation  to  crops 497 

Air 432 

Amendments 348 

Analysis,    influence    of    absorp- 
tion   277 

Broader  than  geology 2 

Characteristics  of 2 

Clay 74 

Cohesiveness 98 

Environment  of  plant 1 

Factors  in  plant  growth 1 

Forming  rocks,  important 9 

Functions  of 1 

Management  and  texture 80 

Mechanism 2 

Moisture,  adjuiitment  of   170 

Number  of  particles  in  classes.  .   82 

Organic  constituent 119 

Organic  matter  in 125 

Organisms 388 

Particles,  constitution  of 69 

Particles,  influence  surface  cool- 
ing on  movement  of  moisture .  183 
Particles,  relation  to  root -bain.  287 


PAOB 

Soil,  plasticity 98 

Productiveness    in    relation    to 

crop  rotation 503 

Productiveness  of  arid  and  humid  69 
Requirements    of    growth    fur- 
nished by 501 

Sandy  74 

Separates,  composition  of 84 

Series,  defined 78 

Structure,  defined    88 

Study,  viewpoint 2 

Surface  area  of 83 

Types 78 

Units 80 

Solar    radiation,    relation    to    soil 

temperature 451 

Soluble  material  in  water  of  chief 

rivers 24 

Solubility  of 'the  soil 267 

Solubility  of  the  soil,  surface  ex- 
posed   267 

Composition  of  particles 267 

Organic  matter 127 

Relation  to  texture 270 

Strength  of  solvent  agents 267 

Soluble  salts,  affect  flocculation.  .  .  116 
Soluble  salts, as  cementing  material  100 

Effect  on  structure 1 16 

Solution,  affect  movement  of  water  173 

By  water  in  rock  decay 22 

Surface  tensions 173 

Sorting  power  of  water 27 

Source  of  soil  material 2 

Specific  gravity  of  soil,  absolute.  .    94 

Of  humus 128 

Table  of  apparent 96 

Specific  heat  of  soil  and  water.  .  .    455 
Spray   irrigation,   advantage*   and 

disadvantage*  of 233 

Spray  irrigation,  condition*  where 

used 232 

Spring  plowing ....  ..210 

Starch,  decomposition  by  bacteria  405 

Stassf urt  salts 340 

Steamed  bone  .  335 


Stewart,  figures  on  capillary  move- 
ment   181 

Effect    of    do.h    shade    on    soil 

moisture 214 

Rainfall  causes  low  of  soil  water  198 
Storer,     quotes     Del  men,     water 

capacity 153 

Reports  soil  moisture  for  crops .  .  155 


528 


INDEX 


PAGE 

Storing  water  in  soil,  importance 

of  depth 198 

Straw  as  litter  in  manure 367 

Straw  mulches,  in  potato  growing. 201 

Stream-formed  soils 47 

Structure  of  soil,  defined 88 

Structure,  conditions  affecting.  .  .  .  103 

Affected  by  animal  life 118 

Affected  by  carbonate  of  lime.  .352 

Affected  by  caustic  lime 352 

Affected  by  crop  rotations 506 

Affects  capillary  water 151 

Affected  by  organic  matter.  . .  .114 

Affected  by  rainfall 119 

Affected  by  soluble  salts 116 

Affected  by  surface  covering.  . .  119 

And  moisture  content 105 

Crumb 81 

Effect  on  capillary  movement.  .182 

Heat  conductivity 459 

Percolation 167 

Tillage  on Ill,  469 

Lime 116 

Nitrate  of  soda 118 

Structure,  effect  of  plow  on 469 

Affected  bjjjopts 13 

Granular  . . . ^ 91 

Ideal  aspects 88 

Influence  of  drainage  on 240 

-Modification  of 104 

•Puddled 90 

Relation  to  texture 88,  92 

Relation  to  soil  air 432 

Relation  to  moisture 156 

Relation  to  porosity 88 

Structure,  separate  grain 89 

Sub-irrigation,  natural 233 

Advantages  and  disadvantages. 234 

Method  of  artificial 234 

"Subsoil",  defined 68 

Organic  matter  in 125 

Plows,  operation  of 218 

Plow,  purpose  of 477 

Plow,  when  to  use 478 

Productiveness     of      arid     and 

humid 69 

Subsoiling,  aims  of 218 

Conditions  requiring 218 

Increases  water  capacity 219 

May  be  injurious 218 

Relation  to  sub-surface  packing  .212 

Time  to  perform 212 

Substitution  of  bases  in  the  soil.  .  .297 


PAGE 

Substitution  of  one  mineral  nutri- 
ent for  another 280 

Packer 488 

Packing  necessary  in  subsoiling .  220 

Sub-surface  packing 212 

Sulfate  of  lime,  effect  on  soil 354 

Sulfate  of  potash 341 

Sulfur,  as  plant-food  element 3 

Sunshine  distribution  of  in  United 

States 452 

Relation  to  soil  temperature  452, 453 
Superphosphates,  composition  of. 338 

Fertilizers 337 

Manufacture  of 337 

Supply  of  soil  water 136 

Surface  area,  in  field  soils 83 

Calculation  of 83 

Relation  to  absorption 103 

Relation  to  texture 82 

Surface  covering,  influence  on  struc- 
ture  119 

Surface  tension.and  capillarityl58,  182 

Affected  by 159,  173 

And   plasticity 97 

Movement  of  water  by 182 

Relation  of  fertilizers  to 182 

Relation  to  granulation  in  tillagell2 
Surface  drains.   See  Open  drains. 
Swamp  soil,  organic  matter  in.  ...  125 
Sweep,  cultivators,  efficiency  of  .  .480 

Swine  manure 370 

Symbiosis,  fixation  of  nitrogen  by  .423 
Systems  of  textural  classification  71,  73 

Table  of  surface  tensions 159 

Tankage 334 

Temperature  affects,  movement  of 

moisture 173 

Of  color  on 456 

Percolation 168 

Surface  tension 173 

Temperature  of  soil.   See,  also.  Heat. 

Affected  by  wind 461 

And  air,  monthly  range 454 

At  different  depths 454 

Daily  range  of 453 

Effect  of  drainage  on 463 

Effect  of  manure  on 464 

Effect  of  moisture  on 461 

Effect  on  chemical  processes .  . .  .451 

Effect  on  conductivity 459 

Effect  upon  movement  of  soil  air  441 
Factors  which  determine 453 


INDEX 


529 


PAGE  PAGE 

Temperature  of  soil,  heat  from  in-  Tillage,  implements  of 468 

tenor  of  earth 452  Influence  on  structure Ill 

Influence  of  drainage  on 242  In  grain  fields 209 

Influence  of  organic  decay 452  In  spring,  effect  on  soil  tempera- 
Influence  of  shelter  on  464                ture 461 

Influence  of  slope  on 458    I        Nature's  method 18 

Means  of  modifying 463  Part  of  soil  management 465 

Relation  to  bacteria 400  :        Practice,  best  for  holding  water. 217 

Relation  to  physical  processes .  .  451    ;        Principles  of 466 

Relation  to  specific  heat 455    !        Relation  to  alkali  control 236 

Relation  to  sunshine 453  Relation  to  irrigation  practice .  .  225 

Sources  of  heat  for 451  Relations  to  structure  and  water 

Tensile  strength  and  plasticity    ...   97  movement 182 

Tent  of  cloth,  effect  on  soil  moist-  Rollers,  packers  and  crushers  for 486 

ure 214  j    Time  elements  in  capillary  move- 
Texture,  denned 70    '            ment 180 

And  moisture  capacity 80  Element   in    movement    of   soil 

And  soil  management 80  moisture 173 

Classification 70  To  apply  irrigation  water 224 

Classification,  agricultural 74  <    Tobacco,  irrigation  of  in  Florida .  .  237 

Determines  structure 92  :    Toxic  substances,  effect  of  lime  on  .  350 

Effect  on  capillary  movement .  .  1 76  Relation  to  crop  rotat  ion 509 

Effect  on  heat  conductivity 459       Trade  value  of  fertilizers 344 

Effect  on  percolation 167        Transported  soils 30 

Groups,  limits  in  size 73  Transported  soils,  agencies  of ....   44 

,' Importance  in  mulch 205  Transporting    power    of     flowing 

'.,In  relation  to  soil  air 432    :  water .^ 24 

Modification  of 87        Tubercles 423 

Relation  to  absorption 103        Types,  soil 78 

Relation  to  capillary  water 144 

Relation  to  composition 87  Underdrains,  arrangement  on  hill 

Relation  to  crops 79  land 262 

Relation  to  gravitational  water.  161  Construction  of 251 

Relation  to  hygroscopic  water.  .  144  Depth  of 252 

Relation  to  movement  of  moist-  Effect  on  soil  air 445 

ure 172  Freezing  of 261 

Relation  to  pore  space 93  Frequency  of 254 

Relation  to  solubility 270  Grade  of 258,263 

Relation  to  shrinkage 98  Importance  of  outlet 261 

Relation  to  surface  area 83  Materials  used  in  construction    .251 

Soil,  relation  to  type  of  plow.  .  .470  Plants  roots  enter,  when.  .  .253,  261 

Thermal  movement  of  soil  moisture  189  Silting   up  of 

Tile  drains.   .See  Underdrains.  Si»e  of 255 

Tillage,  objects  of 467  Systems  of  arrangement 254 

Action  of  implements  in 409  Typos  of  tile  used  in   258 

And  granulation Ill         Units  of  soil,  defined .80 

As  means  of  drainage .  .               .  .  263  Units,  used  in  measuring  irrigation 

By  cultivators 478  water 

By  plows .  .  409        Use  of  lime  on  soils 

Early  and  late  to  kill  weeds.  .  .  .491        Urine  in  manures 366 

Effects  on  the  soil 468 

Effect  on  volume  and  movement  Vegetables,  absorption  of  nutiient»296 

of  soil  air 444  View  poiiit   of  soi"  study. 

HH 


530 


INDEX 


PAOE 

Volume  change  of  humus 128 

Of  soil  air,  methods  for  modify- 
ing  443 

Of  water  held  by  soils 154 

Weight,  table  of 96 

Wagner,  on  effect  of  manure  on  soil 

temperature 464 

Warington,  effect  of  lime  on  clay 

soil 117 

Figures  on  water  used  by  plants .  133 
Quotes  figures  on  heat  conduc- 
tivity  459 

Quotes  figures  on  specific  heat .  .  456 
Quotes  on  adequate  soil  moist- 
ure  156 

Quotes,  on  shrinkage 98 

Shrinkage  of  clay  and  humus.  .  .  128 

Specific  gravity  of  humus 128 

Suggestions  on  structure 103 

Warm  soils,  meaning  of 462 

Water,  amount  of  used  by  plants.  133 

Amount  in  soil 135 

As  agency  of  rock  decay 21 

As  agency  of  soil  transportation .   45 

Available  in  field  soils 157 

Capillary  movement  of 169 

Capacity,  maximum 161 

Capacity  of  humus 130 

Capacity  of  soil 136 

Capacity  of 'soil,  effect  of  texture 

and  structure  on 216 

Capacity   of   soil    increased   by 

subsoiling 219 

Capacity  of  soil,  influence  of 

drainage  on 241 

Capacity  of  soil,  means  of  increas- 
ing  216 

Carrying  power 24 

Causes  expansion  of  soil 162 

Character  of   soils  deposited  by.  46 
Chemical  action  of  in  rock  decay  21 
Content  of  soil,  effect  on  temper- 
ature  461 

Content,  statement  of 136 

Content  of  soil,  relation  to  crops  .155 
Composition   of  soils   deposited 

by 49,60,51,52,53 

Critical  content 155 

Division  of  soils  deposited  by ...   46 

Erosion  by 494 

Extract  of  soil 276 

Film ..146 


PAGE 

Water,  films  and  checking  of  soil .  98 

Films  and  structure 105 

Forms  of 141 

Function  of  in  plants 133 

Gravitational   form 160 

Increased  loss  through  rainfall .  198 

In  soil,  availability 141 

In  soil,  density  of 290 

In  soil,  means  of  decreasing 238 

In  soil,  effect  on  movement  of 

air 440 

Lack    of,    often    controls    crop 

yields 197 

Loss  of  from  soil 164 

Material    transported    in    chief 

rivers 26 

Maximum  content 155 

Minimum  content 155 

Mechanical    action    of    in    rock 

decay 24 

Movement  of  in  soil 165 

Soil  as  reservoir 133 

Soil  supply  dependent  on 136 

Solvent  action 21 

Sorting  power 27 

Supply  of 136 

Used  by  different  crops .  .•. 185 

Volume  of,  held 154 

Way,  experiments  with  soil  absorp- 
tion   298 

Weed,  defined 489 

Control  of,  principles 490 

Deplete  soil  moisture 195 

Early  and  late  tillage  to  kill.  ..  .491 

How,  injurious 490 

Implements  useful  to  kill 492 

Relation  to  crop  rotation 508 

Removal  of  saves  water 216 

Special  methods  of  control 492 

Weeders,  type  of  cultivator 483 

Weight  of  soil 94 

Weight  of  soil,  per  cubic  foot 96 

Per  acre-foot 96 

Weight  of  organic  matter  in  soil .  .  128 

Weight  of  peat  and  muck 128 

Wet  soil,  organic  matter  in 125 

White  alkali 309 

Whitney  and  Hosmer,  soil  moist- 
ure   157 

Suggestions  on  structure 103 

Surface  tension  of  solutions. '.  .  .  159 
Wind-blown    material,    character- 
istic of 18 


INDEX 


531 


PAGE 

Windbreaks,  disadvantages  of  . .  .213 

Effectiveness  of 213 

To  prevent  erosion 496 

Wind,  control  of  erosion  by 496 

Deposits,  composition  of 63 

Effect  of  cloth  tent  on 215 

Effect  on  evaporation 213 

Effect  on  percolation 168 

Effect  upon   movement  of  soil 

air 443 

Formed  soils,  deposition  of 60 

Formed  soils,  relation  to  loess. .  60 


PAGE 

Wollny,  best  soil  moisture  content  .156 
Conclusions  concerning  soil  tem- 
perature   462 

On  temperature  of  soil 459 

Water  used  by  plants 134 

Wood  ashes 342 

Worms,  effect  on  the  coil 389 

Zeolites,  effect  on  absorption   by 
the  soil 299 


CYCLOPEDIA  OF  AMERICAN  AGRICULTURE 
Edited  by  L.  H.  Bailey 

Director  of  College  of  Agriculture,  Cornell   University 
and  Profeuor  of  Rural  Economy 

With  100  Full-page  Plates  and  More  Than  2,000  Illustrations  in  the 

Text— Four  Volumes— The  Set:  Cloth,  $20  net— Half 

Morocco,  $32  net — Carriage  Extra 

Volume  I  —  Farms 

The  Agricultural  Regions — The  Projecting  of  a  Farm — The  Soil 
Environment — The  Atmosphere  Environment. 

Volume  II  —  Crops 

The  Plant  and   Its  Relations — The  Manufacture  of  Crop  Prod- 
ucts— North  American  Field  Crops. 

Volume  III  —  Animals 

The  Animal  and   Its   Relations — The   Manufacture  of  Animal 
Products — North  American  Farm  Animals. 

Volume  IV — The  Farm  and  the  Community 

Economics  —  Social  Questions  —  Organizations  —  History —  Lit- 
erature, etc. 

"Indispensable  to  public  and  reference  libraries  .  .  .  readily  compre- 
hensible to  any  person  of  average  education." — The  \atinn. 

"The  completes!  existing  thesaurus  of  up-to-date  facts  and 'opinion* 
on  modern  agricultural  methods.  It  is  safe  to  say  that  many  years 
must  pass  before  it  can  be  surpassed  in  comprehensiveness,  accuracy, 
practical  value,  and  mechanical  excellence.  It  ought  to  be  in  every 
library  in  the  country." — Rrcord  Herald,  Chicago. 


Published  by 

THE  MACMILLAN   COMPANY 

64-66   FIFTH   AVENUE  NEW   YORK 


BOOKS  ON  AGRICULTURE 


On  Selection  of  Land,  etc. 

NET 

Thomas  F.  Hunt 's  How  to  Choose  a  Farm $1  75 

E.  W.  Hilgard's  Soils:  Their  Formation,  and  Relations  to 

Climate  and  Plant  Growth   4  00 

Isaac  P.  Roberts'  The  Farmstead 1  50 

On  Tillage,  etc. 

F.  H.  King's  The  Soil 1  50 

Isaac  P.  Roberts'  The  Fertility  of  the  Land 1   50 

Elwood  Mead's  Irrigation  Institutions 1  25 

F.  H.  King's  Irrigation  and  Drainage 1  50 

William  E.  Smythe  's  The  Conquest  of  Arid  America  ....  1  50 

Edward  B.  Voorhees'  Fertilizers 1  25 

Edward  B.  Voorhees'  Forage  Crops 1  50 

H.  Snyder's  Chemistry  of  Plant  and  Animal  Life 1  25 

H.  Snyder's  Soil  and  Fertilizers.    Third  edition   1  25 

L.  H.  Bailey's  Principles  of  Agriculture 1  25 

W.  C.  Welborn's  Elements  of  Agriculture,  Southern  and 

Western   7t5 

On  Plant  Diseases,  etc. 

George  Massee  's  Plant  Diseases 1  60 

E.  C.  Lodeman's  The  Spraying  of  Plants    1  25 

H.  M.  Ward 's  Diseases  in  Plants  (English) 1  60 

A.  S.  Packard 's  A  Text-book  on  Entomology 4  50 

On  Production  of  New  Plants 

L.  H.  Bailey's  Plant-Breeding 1  25 

L.  H.  Bailey's  The  Survival  of  the  Unlike 2  00 

L.  H.  Bailey's  The  Evolution  of  our  Native  Fruits   2  00 

W.  S.  Harwood's  New  Creations  in  Plant  Life 1  75 

On  Garden-Making 

Bailey  and  Hunn's  Practical  Garden  Book 1  00 

L.  H.  Bailey 's  Garden  Making 1  50 

L.  H.  Bailey's  Vegetable-Gardening 1  50 

L.  H.  Bailey 's  Horticulturist 's  Rule  Book 75 

.      L.  H.  Bailey's  Forcing  Book 1  25 

A.  French's  Book  of  Vegetables    ,  1  75 


BOOKS   ON  AGRICULTURE  —  Continued 

On  Fruit-Growing,  etc.  NET 

L.  H.  Bailey's  Nursery  Book  ..........................  $1  50 

L.  H.  Bailey's  Fruit-Growing  ..........................  1   50 

L.  H.  Bailey  's  The  Pruning-  Book  ......................  1   50 

F.  W.  Card's  Bush  Fruits  ............................  1  50 

On  the  Care  of  Live-stock 

Nelson  S.  Mayo's  The  Diseases  of  Animals  ..............  1  50 

W.  H.  Jordan's  The  Feeding  of  Animals     ..............  1   50 

I.  P   Robert's  The  Horse    ............................  1   25 

George  C.  Watson's  Farm  Poultry   ...................  '  1  25 


On  Dairy  Work,  Farm  Chemistry,  etc. 

Henry  H.  Wing's  Milk  and  Its  Products 


('.  M.  Aikman's  Milk 


Harry  Snyder's  Dairy  Chemistry 

W.    D.    Frost's    laboratory   Guide    in    Klemmtary    Bnc- 


teriology 


50 

25 

00 


GO 


J.  G.  Lipman's  Bacteria  and  Country  Life 

Lincoln  and  Walton's  Elementary  Quantitative  Chemical 

Analysis   1   .50 

On  Economics  and  Organization 

Henry  C.  Taylor's  Agricultural  Economics I  25 

I.  P.  Rol>erts'  The  Farmer's  Business  Handl>ook  1  25 

George  T.  Fairchild's  Hural  Wealth  and  Welfare  1  25 

S.  E.  Sparling's  Business  Organization  1  25 

In  Ihr  Ciliten't  Library.     Inrlmlm  <i  Chni>trr  mi  l-'iirmini). 

L.  H.  Bailey's  The  State  and  tin-  Farmer  1   25 

On  Everything  Agricultural 

L.  H.  Bailey's  Cyclopedia  of  American  Agriculture.    Com- 
plete   in    four   royal    Svo   volumes,   \\ith   over   2(MH) 

illustrations 

Price  of  sets:  Cloth,  $20  net:  half  monxvo.  £12  not 


For  further  information  ••  lo  «njr  of  the  «ho»p,  nddrra*  th«   puhlnhrr. 

THE  MACMILLAN   COMPANY 

64-66    FIFTH   AVENUE  NEW    YORK 


KATE  V.  SAINT  MAUR'S 

A  Self-Supporting  Home 

Cloth,  illustrated,  1 2mo,  $1 .75  net 

"If  the  reader  has  in  him  the  least  spark  of  love  for  country  life,  this 
book  will  fan  it  into  such  a  glow  that  he  will  be  possessed  of  but  one  am- 
bition— to  establish  a  self-supporting  home  of  his  own.  .  .  .  The  book 
is  full  of  explicit  practical  helps  on  all  kinds  of  farm  industries,  and  the 
amateur  may  choose  the  two  or  three  subjects  most  to  his  liking  and 
gain  from  these  pages  valuable  information."— JVje  N.  Y.  Eaemny  Post. 

The  Earth's  Bounty 

Cloth,  illustrated,  I2mo,  $1 .75  net 

"Every  line  is  instinct  with  the  personality  of  a  practical  and  devoted 
farmer,  while  the  book  has  the  charm  of  a  vigorous  Imagination  and 
i          forceful  mind.   The  photographic  illustrations  are  exceptionally  good — 
many  of  them  very  beautiful."— The  Independent. 

MR.  BOLTON  HALL'S 

Three  Acres  and  Liberty 

Cloth,  illustrated,  1 2mo.  $1.75  net 

The  author  discusses  the  possibilities  of  an  acre;  where  to  find  idle 
land;  how  to  select,  clear  and  cultivate  it;  the  results  to  be  expected; 
what  an  acre  may  produce,  and  nearly  every  other  imaginable  topic  of 
intensive  farming  in  clear,  definite  statements  which  are  easily  veri- 
fied. It  is  a  practical  book  from  cover  to  cover. 

JOHN  WILLIAM  STREETER'S 
The  Fat  of  the  Land 

Cloth,  I2mo,$l  .50  net 

It  tells  what  can  be  accomplished  through  the  application  of  business 
methods  to  the  farming  business.  Never  was  the  freshness,  the  beauty, 
the  freedom,  the  joy  of  country  life,  put  in  a  more  engaging  fashion. 

W.  S.  HARWOOD'S 
The  New  Earth 

A  Recital  of  the  Triumph*  of  Modern  Agriculture  in  America 

Cloth,  illustrated,  1 2mo,  $1.75  net 

Mr.  Harwood  shows  in  a  very  entertaining  way  the  remarkable  prog- 
ress of  the  past  two  generations  along  all  the  lines  which  have  their 
focal  point  in  the  earth. 

New  Creations  in  Plant  Life 

An  Account  of  the  Life  and  Work  of   Luther  Burbank 

Cloth,  1 2 mo,  $1.75  net 

"By  far  the  most  successful  account  we  have  yet  seen  of  Burbank's 
successful  labors."— The  New  fork  Ereninu  Post. 

THE  MACMILLAN  CO.,  64-66  Fifth  Avenue,  New  York 


OF  CALIFOr 


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•RARY 


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001  096  053     2 


